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  • 1. Aad, G.
    et al.
    Buszello, Claus P.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Costa, Marcio Jorge Teles da
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ekelöf, Tord
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Ellert, Mattias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Ferrari, Arnaud
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Isaksson, Charlie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Pelikan, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Zwalinski, L.
    Search for anomalous production of prompt like-sign lepton pairs at root s=7 TeV with the ATLAS detector2012In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, Vol. 12, p. 007-Article in journal (Refereed)
    Abstract [en]

    An inclusive search for anomalous production of two prompt, isolated leptons with the same electric charge is presented. The search is performed in a data sample corresponding to 4.7 fb(-1) of integrated luminosity collected in 2011 at root s = 7TeV with the ATLAS detector at the LHC. Pairs of leptons (e(+/-)e(+/-), e(+/-)mu(+/-), and mu(+/-)mu(+/-)) with large transverse momentum are selected, and the dilepton invariant mass distribution is examined for any deviation from the Standard Model expectation. No excess is found, and upper limits on the production cross section of like-sign lepton pairs from physics processes beyond the Standard Model are placed as a function of the dilepton invariant mass within a fiducial region close to the experimental selection criteria. The 95% confidence level upper limits on the cross section of anomalous e(+/-)e(+/-), e(+/-)mu(+/-), or mu(+/-)mu(+/-) production range between 1.7 fb and 64 fb depending on the dilepton mass and flavour combination.

  • 2.
    Adler, Severino
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Hubbard to Falicov-Kimball-a DMFT[DMRG] study2017Student paper other, 10 credits / 15 HE creditsStudent thesis
  • 3.
    Ahuja, Rajeev
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Blomqvist, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Larsson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Pyykkö, Pekka
    Zaleski-Ejgierd, Patryk
    Relativity and the lead–acid battery2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, no 1, p. 018301-Article in journal (Refereed)
    Abstract [en]

    The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at nonrelativistic, scalar-relativistic, and fully relativistic levels, and using several exchange-correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7-1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

  • 4. Airiskallio, E
    et al.
    Nurmi, E
    Heinonen, M H
    Vayrynen, I J
    Kokko, K
    Ropo, M
    Punkkinen, M P J
    Pitkanen, H
    Alatalo, M
    Kollar, J
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Third element effect in the surface zone of Fe-Cr-Al alloys2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 3, p. 033105-Article in journal (Refereed)
    Abstract [en]

    The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.

  • 5. Airiskallio, E.
    et al.
    Nurmi, E.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Johansson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Magnetic origin of the chemical balance in alloyed Fe-Cr stainless steels: First-principles and Ising model study2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 92, p. 135-140Article in journal (Refereed)
    Abstract [en]

    Iron-chromium is the base material for most of the stainless steel grades. Recently, new insights into the origins of fundamental physical and chemical characteristics of Fe-Cr based alloys have been achieved. Some of the new results are quite unexpected and call for further investigations. The present study focuses on the magnetic contribution in the atomic driving forces related to the chemical composition in Fe-Cr when alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab initio exact muffin-tin orbitals method combined with an Ising-type spin model, we demonstrate that the magnetic moment of the solute atoms with the induced changes in the magnetic moments of the host atoms form the main factor in determining the mixing energy and chemical potentials of low-Cr Fe-Cr based alloys. The results obtained in the present work are related to the designing and tuning of the microstructure and corrosion protection of low-Cr steels. (C) 2014 Elsevier B. V. All rights reserved.

  • 6. Alam, Aftab
    et al.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mookerjee, Abhijit
    Effect of disorder on the electronic properties of graphene: A theoretical approach2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 8, p. 085454-Article in journal (Refereed)
    Abstract [en]

    In order to manipulate the properties of graphene, it is very important to understand the electronic structure in the presence of disorder. We investigate, within a tight-binding description, the effects of disorder in the on-site (diagonal disorder) term in the Hamiltonian as well as in the hopping integral (off-diagonal disorder) on the electronic dispersion and density of states by the augmented space recursion method. Extrinsic off-diagonal disorder is shown to have dramatic effects on the two-dimensional (2D) Dirac cone, including asymmetries in the band structures as well as the presence of discontinuous bands (because of resonances) in certain limits. Disorder-induced broadening, related to the scattering length (or lifetime) of Bloch electrons, is modified significantly with increasing strength of disorder. We propose that our methodology is suitable for the study of the effects of disorder in other 2D materials, such as a boron nitride monolayer.

  • 7.
    Albaalbaky, Ahmed
    et al.
    Normandie Univ, UNIROUEN, CNRS, INSA Rouen,GPM, St Etienne Du Rouvray, France..
    Kvashnin, Yaroslav
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ledue, Denis
    Normandie Univ, UNIROUEN, CNRS, INSA Rouen,GPM, St Etienne Du Rouvray, France..
    Patte, Renaud
    Normandie Univ, UNIROUEN, CNRS, INSA Rouen,GPM, St Etienne Du Rouvray, France..
    Fresard, Raymond
    Normandie Univ, UNICAEN, ENSICAEN, CNRS,CRISMAT, F-14050 Caen, France..
    Magnetoelectric properties of multiferroic CuCrO2 studied by means of ab initio calculations and Monte Carlo simulations2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 6, article id 064431Article in journal (Refereed)
    Abstract [en]

    Motivated by the discovery of multiferroicity in the geometrically frustrated triangular antiferromagnet CuCrO2 below its Neel temperature T-N, we investigate its magnetic and ferroelectric properties using ab initio calculations and Monte Carlo simulations. Exchange interactions up to the third nearest neighbors in the ab plane, interlayer interaction, and single ion anisotropy constants in CuCrO2 are estimated by a series of density functional theory calculations. In particular, our results evidence a hard axis along the [110] direction due to the lattice distortion that takes place along this direction below T-N. Our Monte Carlo simulations indicate that the system possesses a Neel temperature T-N approximate to 27 K very close to the ones reported experimentally (T-N = 24-26 K). Also we show that the ground state is a proper-screw magnetic configuration with an incommensurate propagation vector pointing along the [110] direction. Moreover, our work reports the emergence of spin helicity below T-N which leads to ferroelectricity in the extended inverse Dzyaloshinskii-Moriya model. We confirm the electric control of spin helicity by simulating P-E hysteresis loops at various temperatures.

  • 8. Ali, Md Ehesan
    et al.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of Noncovalent Cation/Anion-pi Interactions on the Magnetic Exchange Phenomenon2011In: The Journal of Physical Chemistry Letters, ISSN 1948-7185, Vol. 2, no 9, p. 939-943Article in journal (Refereed)
    Abstract [en]

    The role of noncovalent ion-pi interactions in controlling the intramolecular magnetic exchange interaction in 1,3-phenylene-based bis(aminoxyl) diradical has been investigated computationally through deploying an external ion in the vicinity of the pi-cloud of the phenylene coupler. Using spin-polarized hybrid density functional theory-based calculations, we observe that the anions drastically enhance the magnetic exchange interaction for distances below the equilibrium distance. The phenomenon could be understood by two simultaneously occurring effects, which influence the intramolecular magnetic exchange interaction. The first one is the enhancement of the paratropic current density on the aryl couplers due to a small amount of charge transfer. The other one is the attainment of magnetization density on the anionic species due to such charge transfer, favorably altering the magnetic exchange pathway. The achieved understanding provides prospects of a completely new strategy of enhancing the intramolecular ferromagnetic coupling through the assistance of external ionic species inserted in molecular crystals.

  • 9.
    Ali, Md. Ehesan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Unraveling the Electronic Structure, Spin States, Optical and Vibrational Spectra of Malaria Pigment2015In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 21, no 23, p. 8544-8553Article in journal (Refereed)
    Abstract [en]

    A detailed knowledge of the electronic structure and magnetic and optical properties of hemozoin, the malaria pigment, is essential for the design of effective antimalarial drugs and malarial diagnosis. By employing state-of-the-art electronic structure calculations, we have performed an in-depth investigation of the malaria pigment. Specifically, molecular bond lengths and spin states of the two Fe-III heme centers and their exchange interaction, the UV/Vis absorption spectrum, and the IR vibrational spectra were calculated and compared with available experimental data. Our density functional theory (DFT)-based calculations predict a singlet ground spin state that stems from an S=5/2 spin state on each of the Fe heme centers with a very weak antiferromagnetic exchange interaction between them. Our theoretical UV/Vis and IR spectra provide explanations for various spectroscopic studies of hemozoin and -hematin (a synthetic analogue of hemozoin). A good comparison of calculated and measured properties demonstrates the convincing unveiling of the electronic structure of the malaria pigment. Based on the predicted vibrational spectra, we propose a unique spectral band from the nuclear resonance vibrational spectroscopy (NRVS) results that could be used as a key fingerprint for malarial detection.

  • 10. Ali, Md. Ehesan
    et al.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Electronic Structure, Spin-States, and Spin-Crossover Reaction of Heme-Related Fe-Porphyrins: A Theoretical Perspective2012In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 116, no 20, p. 5849-5859Article in journal (Refereed)
    Abstract [en]

    The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a "double spin-crossover" feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).

  • 11.
    Ali, Md Ehesan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Staemmler, Volker
    Illas, Francesc
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Designing the Redox-Driven Switching of Ferro- to Antiferromagnetic Couplings in Organic Diradicals2013In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 9, no 12, p. 5216-5220Article in journal (Refereed)
    Abstract [en]

    Switching of the magnetic exchange coupling from ferro- to antiferromagnetic or vice versa in a single molecule is an appealing but rarely occurring phenomenon in molecular magnetism. Here, we report this for an unprecedented pure organic system, computationally designed by tailoring a conformationally restricted, known nitroxide-diradical (Rajca et al. J. Am. Chem. Soc. 2007, 129, 10159). This ferro- to antiferromagnetic coupling switching of an "m-phenylene" based diradical is governed by a stereoelectronic effect and controlled by a redox-driven chemical reaction.

  • 12. Almeida, Cristiane G.
    et al.
    Araujo, Rafael B.
    Yoshimura, Rafael G.
    Mascarenhas, Artur J. S.
    da Silva, Antonio Ferreira
    Araujo, Carlos Moyses
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Silva, Luciana A.
    Photocatalytic hydrogen production with visible light over Mo and Cr-doped BiNb(Ta)O-42014In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 39, no 3, p. 1220-1227Article in journal (Refereed)
    Abstract [en]

    In the present work, we prepared pure and Cr(III) and Mo(V)-doped BiNbO4 and BiTaO4 by the citrate method. Pure BiNbO4 and BiTaO4 were obtained in triclinic phase at 600 degrees C and 800 degrees C, respectively. The metal doping influenced strongly the crystal structure as well as the photocatalytic activity of the oxides. The XRD data could prove that the Mo(V) doping induces the orthorhombic phase, while the Cr(III) doping favors the triclinic phase for both oxides. Metal doping also modified the photosensitivity of the oxides, extending the absorption toward the visible light region. The photocatalytic activity in water splitting under visible light irradiation was evaluated by monitoring the H-2, CO2 and CO evolution. The results showed that Cr(III)-doped BiTaO4 and BiNbO4, in general, are more selective for hydrogen production, while Mo(V)-doped materials are more selective for CO2 generation. Comparing the photocatalytic activity of BiTaO4 and BiNbO4, the former shows higher activity for hydrogen production as well as for CO2 generation, specially when the concentration was 2% in Cr(III) and Mo(V), respectively. Those results are in agreement with the computational study to access the effect of doping on the electronic structure. For Mo(V)-doped materials a negligible change of conduction band minimum potential was found, indicating that there might be no improvement on the reduction power of the material following the substitutional doping. In Cr(III)-doped BiNbO4 there is a slight shift of the CBM potential increasing a little bit the reduction power. However, the effect is much stronger in the Cr(III)-doped BiTaO4.

  • 13.
    Almeida, Roseley
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Univ Fed Bahia, Inst Fis, Campus Univ Ondina, Salvador, BA, Brazil.
    Banerjee, Amitava
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Almeida, Jailton
    Univ Fed Bahia, Inst Fis, Campus Univ Ondina, BR-40210340 Salvador, BA, Brazil..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, Stockholm, Sweden.
    Theoretical Evidence behind Bifunctional Catalytic Activity in Pristine and Functionalized Al2C Monolayers2018In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 19, no 1, p. 148-152Article in journal (Refereed)
    Abstract [en]

    First principles electronic structure calculations based on the density functional theory (DFT) framework are performed to investigate hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on two-dimensional Al2C monolayers. In addition to the pristine Al2C monolayer, monolayers doped with Nitrogen (N), Phosphorous (P), Boron (B), and Sulphur (S) are also investigated. After determining the individual adsorption energy of hydrogen and oxygen on the different functionalized Al2C monolayers, the adsorption free energies are predicted for each of the functionalized monolayers in order to assess their suitability for HER or OER. The density of states and optical absorption spectra calculations along with the work function of the functionalized Al2C monolayers enable us to gain a profound understanding of the electronic structure for the individual system and their relation to the water splitting mechanism.

  • 14. Al-Zoubi, N I
    et al.
    Punkkinen, M P J
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Completeness of the exact muffin-tin orbitals: Application to hydrogenated alloys2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 4, p. 045122-Article in journal (Refereed)
    Abstract [en]

    We investigate the basis set convergence of the exact muffin-tin orbitals by monitoring the equation of state for Al, Cu, and Rh calculated in the conventional face-centered-cubic lattice (str-I) and in a face-centered-cubic lattice with one atomic and three empty sites per primitive cell (str-II). We demonstrate that three (spd) muffin-tin orbitals are sufficient to describe Al in both structures, but for str-II Cu and Rh at least five (spdfg) orbitals are needed to get converged equilibrium Wigner-Seitz radius (within <= 0.8%) and bulk modulus (<= 3.3%). We ascribe this slow convergence to the nearly spherical densities localized around the Cu and Rh atoms, which create strongly asymmetric charge distributions within the nearest cells around the empty sites. The potential sphere radius dependence of the theoretical results for structure str-II is discussed. It is shown that a properly optimized overlapping muffin-tin potential in combination with the spdfg basis yields acceptable errors in the equilibrium bulk properties. The basis set convergence is also shown on hydrogenated Sc and Sc-based alloys.

  • 15. Al-Zoubi, N.
    et al.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nilson, G.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    The Bain path of paramagnetic Fe-Cr based alloys2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 1, p. 013708-Article in journal (Refereed)
    Abstract [en]

    Employing the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, we calculated the total energy and local magnetic moments of paramagnetic Fe-Cr-M (M = Cr, Mn, Fe, Co, Ni) alloys along the tetragonal distortion (Bain) path connecting the body centered cubic (bcc) and the face centered cubic (fcc) structures. The paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all alloys, the local magnetic moments on Fe atoms decrease from the maximum value corresponding to the bcc phase toward the minimum value realized for the fcc phase. Cobalt atoms have non-vanishing local magnetic moments only for tetragonal lattices with c/a < 1.30, whereas the local magnetic moments of Mn show weak crystal structure dependence. We find that Cr stabilizes the bcc lattice and increases the energy barrier as going from the bcc toward the fcc phase. Both Co and Ni favor the fcc lattice and decrease the energy barrier relative to the bcc phase. On the other hand, the tetragonal distortion around the fcc phase is facilitated by Cr and to a somewhat lesser extent also by Ni, but strongly impeded by Co. Manganese has negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. Our findings on the alloying induced softening or hardening of Fe-Cr based alloys against tetragonal distortions are important for understanding the interstitial driven martensitic transformations in alloy steels.

  • 16. Al-Zoubi, N.
    et al.
    Punkkinen, M. P. J.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of magnesium on hydrogenated ScAl(1-x)Mg(x) alloys: A theoretical study2011In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, no 10, p. 2848-2853Article in journal (Refereed)
    Abstract [en]

    Ab initio total energy calculations, based on the projector augmented wave method and the exact muffin-tin orbitals method in combination with the coherent-potential approximation, are used to examine the effect of magnesium on hydrogen absorption/desorption temperature and phase stability of hydrogenated ScAl(1-x)Mg(x) (0 <= x <= 0.3) alloys. According to the experiments, ScAl(1-x)Mg(x) adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH(2) with CaF(2) structure and Al-Mg with face centered cubic structure. Here we demonstrate that the stability field of the hydrogenated alloys depends sensitively on Mg content and on the microstructure of the decomposed system. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with Mg concentration.

  • 17.
    Al-Zoubi, N.
    et al.
    Tafila Tech Univ, Dept Appl Phys, Tafila, Jordan..
    Schönecker, S.
    KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, Stockholm, Sweden..
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, Stockholm, Sweden..
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, Stockholm, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, Budapest, Hungary..
    Assessing the Exact Muffin-Tin Orbitals method for the Bain path of metals2017In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 97, no 15, p. 1243-1264Article in journal (Refereed)
    Abstract [en]

    We scrutinise the muffin-tin approximation and the screening within the framework of the Exact Muffin-Tin Orbitals method in the case of cubic and tetragonal crystal symmetries. Systematic total energy calculations are carried out for the Bain path including the body-centred cubic and face-centred cubic structures for a set of simple and transition metals. The present converged results in terms of potential sphere radius (S) and hard sphere radius (b) are in good agreement with previous theoretical calculations. We demonstrate that for all structures considered here, potential sphere radii around and slightly larger than the average Wigner-Seitz radius (w) yield accurate total energy results whereas S values smaller than w give large errors. It is shown that for converged total energies hard spheres with radii b = 0.7-0.8w should be used for an efficient screening within real space clusters consisting typically of 70-90 lattice sites. The less efficient convergence of the total energy in the case of small hard spheres is ascribed to the delocalisation of the screened spherical waves, which leads to inaccurate interstitial overlap matrix. The above conclusions are not significantly affected by the volume of the system.

  • 18. Al-Zoubi, N.
    et al.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Medvedeva, A.
    Andersson, J.
    Nilson, G.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Tetragonality of carbon-doped ferromagnetic iron alloys: A first-principles study2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 1, p. 014112-Article in journal (Refereed)
    Abstract [en]

    Using density-functional theory in combination with the exact muffin-tin orbital (EMTO) method and coherent potential approximation, we investigate the alloying effect on the tetragonality of Fe-C solid solution forming the basis of steels. In order to assess the accuracy of our approach, first we perform a detailed study of the performance of the EMTO method for the Fe(16)C(1) binary system by comparing the EMTO results to those obtained using the projector augmented wave method. In the second step, we introduce different substitutional alloying elements (Al, Cr, Co, Ni) into the Fe matrix and study their impact on the structural parameters. We demonstrate that a small amount of Al, Co, and Ni enhances the tetragonal lattice ratio of Fe(16)C(1) whereas Cr leaves the ratio almost unchanged. The obtained trends are correlated with the single-crystal elastic parameters calculated for carbon-free alloys.

  • 19. Al-Zoubi, Noura
    et al.
    Li, Xiaoqing
    Schonecker, Stephan
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of manganese on the bulk properties of Fe-Cr-Mn alloys: a first-principles study2014In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 89, no 12, p. 125702-Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of manganese on lattice stability and magnetic moments of paramagnetic Fe-Cr-Mn steel alloys along the Bain path connecting the body-centered cubic (bcc) and face-centered cubic (fcc) structures. The calculations are carried out using the ab initio exact muffin-tin orbital method, in combination with the coherent potential approximation, and the paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all Fe-Cr-Mn alloys considered here, the local magnetic moments on Fe atoms have the minimum values for the fcc structure and the maximum values for the bcc structure, whereas the local magnetic moments on Mn have almost the same value along the constant-volume Bain path. Our results show that Mn addition to paramagnetic Fe-Cr solid solution stabilizes the bcc structure. However, when considering the paramagnetic fcc phase relative to the ferromagnetic bcc ground state, then Mn turns out to be a clear fcc stabilizer, in line with observations.

  • 20. Amft, M.
    et al.
    Walle, L. E.
    Ragazzon, Davide
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Borg, A.
    Uvdal, P.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sandell, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    A Molecular Mechanism for the Water-Hydroxyl Balance during Wetting of TiO22013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 33, p. 17078-17083Article in journal (Refereed)
    Abstract [en]

    We show that the formation of the wetting layer and the experimentally observed continuous shift of the H2O-OH balance toward molecular water at increasing coverage on a TiO2(110) surface can be rationalized on a molecular level. The mechanism is based on the initial formation of stable hydroxyl pairs, a repulsive interaction between these pairs, and an attractive interaction with respect to water molecules. The experimental data are obtained by synchrotron radiation photoelectron spectroscopy and interpreted with the aid of density functional theory calculations and Monte Carlo simulations.

  • 21.
    Amft, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Density Functional Theory Studies of Small Supported Gold Clusters and Related Questions: What a Difference an Atom Makes2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    During the last decades the specific manipulation of matter on the (sub-) nanometer scale, also known as nanoscience, became possible by technologies such as the scanning tunneling microscope. Nanocatalysts, i.e. catalytic active structures of up to a few nanometers in size, belong to this rather new class of materials. Unlike ordinary ’macroscopic’ catalytic materials, the performance of nanocatalysts does not simply scale, for instance, with the surface to volume ratio of the active material.

    In this Thesis model nanocatalysts are investigated by means of ab-initio density functional theory calculations.

    In paper I, we explain the experimentally observed catalytic characteristics of small gold clusters, Au1-4, on a regular magnesium oxide terrace towards the oxidation of carbon monoxide by thoroughly studying the adsorption of CO and O2 on these clusters.

    In the subsequent paper II, we study the feasibility of a catalytic water-mediated CO oxidation reaction on Au1-4/MgO and find that this reaction mechanism is not assessable for Au2,4/MgO and unlikely for Au1,3/MgO.

    Papers III and IV concentrate on the reactivity of clusters in the gas phase. Particularly, we focus on the relative stability of Au13 isomers and its potential for O2 dissociation (paper III). We find the lowest energy isomers, which contain a triangular prism at their center surrounded by a ring of the remaining seven atoms, to be generally stable upon O2 adsorption. The dissociation of O2 at certain sites of Au13 is found to be exothermic.

    In paper IV we performed scans of the Born-Oppenheimer potential energy surfaces of neutral and charged Cu3, Ag3, and Au3 to explore the thermally excited vibrations of these trimers. While the Born-Oppenheimer surface of Cu3 exhibits one fairly deep energy minimum, it is comparatively flat with two shallow minima in the case of Ag3. Hence for Ag3 there exist many thermally accessible geometries in a wide range of angles and bond lengths. For Au3, two distinct energy minima appear, being well-separated by a barrier of 180 meV. Already at room temperature, we find bond lengths changes of up to 5% for the studied trimers. Choosing Au3 as a case study for the changed reactivity of thermally excited modes, we find CO to bind up to 150 meV stronger to the excited cluster.

    Gold deposited on graphene and graphite was observed to form larger aggregates. In paper V, we study the electronic structures, high mobility, and substrate-mediated clustering processes of Au1-4 on graphene.

    Already in the 1970s is was speculated that dispersion forces, i.e. van der Waals forces, significantly contribute to the adsorption energies of gold atoms on graphite. We accounted for van der Waals interactions in our density functional theory calculations (paper VI) and investigated the influence of these dispersion forces on the binding of copper, silver, and gold adatoms on graphene. While copper and gold show a mixed adsorption mechanism, i.e. chemical binding plus attraction due to the van der Waals forces, silver is purely physisorbed on graphene.

  • 22.
    Amft, Martin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Thermally Excited Vibrations in Copper, Silver, and Gold Trimers and Enhanced Binding of CO2010In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126Article in journal (Other academic)
  • 23.
    Amft, Martin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Johansson, Börje
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of the cluster dimensionality on the binding behavior of CO and O(2) on Au(13)2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 2, p. 024312-Article in journal (Refereed)
    Abstract [en]

    We present an ab initio density functional theory study of the binding behavior of CO and O(2) molecules to two-and three-dimensional isomers of Au(13) in order to investigate the potential catalytic activity of this cluster towards low-temperature CO oxidation. First, we scanned the potential energy surface of Au(13) and studied the effect of spin-orbit coupling on the relative stabilities of the 21 isomers we identified. While spin-orbit coupling increases the stability of the three-dimensional more than the two-dimensional isomers, the ground state structure at 0 K remains planar. Second, we systematically studied the binding of CO and O(2) molecules onto the planar and three-dimensional structures lowest in energy. We find that the isomer dimensionality has little effect on the binding of CO to Au(13). O(2), on the other hand, binds significantly to the three-dimensional isomer only. The simultaneous binding of multiple CO molecules decreases the binding energy per molecule. Still, the CO binding remains stronger than the O(2) binding. We did not find a synergetic effect due to the co-adsorption of both molecular species. On the three-dimensional isomer, we find O(2) dissociation to be exothermic with an dissociation barrier of 1.44 eV.

  • 24.
    Amft, Martin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lebègue, Sébastien
    Laboratoire de Cristallographie, Résonance Magnétique et Modélisations, Institut Jean Barriol, Nancy Université, Vandoeuvre-lès-Nancy, France.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 39, article id 395001Article in journal (Refereed)
    Abstract [en]

    We performed a systematic density functional (DF) study of the adsorption of copper, silver, and gold adatoms on pristine graphene, especially accounting for van der Waals (vdW) interactions by the vdW-DF and PBE + D2 methods. In particular, we analyze the preferred adsorption site (among top, bridge, and hollow positions) together with the corresponding distortion of the graphene sheet and identify diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to the graphene sheet and that in some cases the buckling of the graphene layer can be significant. Only the results for silver are qualitatively at variance with those obtained with the generalized gradient approximation, which gives no binding in this case. However in all three cases, we observe some quantitative differences between the vdW-DF and PBE + D2 methods. For instance the adsorption energies calculated with the PBE + D2 method are systematically higher than the ones obtained with vdW-DF. Moreover, the equilibrium distances computed with PBE + D2 are shorter than those calculated with the vdW-DF method.

  • 25.
    Amft, Martin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skorodumova, Natalia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Small gold clusters on graphene, their mobility and clustering: a DFT study2011In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 23, no 20, p. 205301-Article in journal (Refereed)
    Abstract [en]

    Motivated by the experimentally observed high mobility of gold atoms on graphene and their tendency to form nanometer-sized clusters, we present a density functional theory study of the ground state structures of small gold clusters on graphene, their mobility and clustering. Our detailed analysis of the electronic structures identifies the opportunity to form strong gold-gold bonds and the graphene-mediated interaction of the pre-adsorbed fragments as the driving forces behind gold's tendency to aggregate on graphene. While clusters containing up to three gold atoms have one unambiguous ground state structure, both gas phase isomers of a cluster with four gold atoms can be found on graphene. In the gas phase the diamond-shaped Au-4(D) cluster is the ground state structure, whereas the Y-shaped Au-4(Y) becomes the actual ground state when adsorbed on graphene. As we show, both clusters can be produced on graphene by two distinct clustering processes. We also studied in detail the stepwise formation of a gold dimer out of two pre-adsorbed adatoms, as well as the formation of Au-3. All reactions are exothermic and no further activation barriers, apart from the diffusion barriers, were found. The diffusion barriers of all studied clusters range from 4 to 36 meV only, and are substantially exceeded by the adsorption energies of -0.1 to -0.59 eV. This explains the high mobility of Au1-4 on graphene along the C-C bonds.

  • 26.
    Amft, Martin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Catalytic activity of small MgO-supported Au clusters towards CO oxidation: A density functional study2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 19, p. 195443-Article in journal (Refereed)
    Abstract [en]

    In order to explain the experimentally found catalytic characteristics of Au1-4 /MgO (100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O2 molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au1-4 /MgO+CO (O2). Experimentally Au1,2 /MgO were found to be inactive under a mixed atmosphere. We show that O2 strongly binds to Au1 /MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au 2-, is energetically possible for Au2 /MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4 /MgO could be explained by their ability to coadsorb CO and O2, hence indicating the occurrence of a Langmuir- Hinshelwood-type reaction mechanism for these clusters.

  • 27.
    Amft, Martin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Does H2O improve the catalytic activity of Au1−4/MgO towards CO oxidation?2010In: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694Article in journal (Other academic)
  • 28.
    Amft, Martin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    The relative stability of Au13 isomers and their potential for O2 dissociationManuscript (preprint) (Other academic)
  • 29.
    Amorim, Rodrigo G.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Univ Fed Fluminense, Dept Fis, ICEx, Volta Redonda, RJ, Brazil..
    Rocha, Alexandre R.
    Univ Estadual Paulista, UNESP, Inst Fis Teor, Sao Paulo, Brazil..
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Boosting DNA Recognition Sensitivity of Graphene Nanogaps through Nitrogen Edge Functionalization2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 34, p. 19384-19388Article in journal (Refereed)
    Abstract [en]

    One of the challenges for next generation DNA sequencing is to have a robust, stable, and reproducible nanodevice. In this work, we propose how to improve the sensing of DNA nucleobase using functionalized graphene nanogap as a solid state device. Two types of edge functionalization, namely, either hydrogen or nitrogen, were considered. We showed that, independent of species involved in the edge passivation, the highest-to-lowest order of the nucleobase transmissions is not altered, but the intensity is affected by several orders of magnitude. Our results show that nitrogen edge tends to p-dope graphene, and most importantly, it contributes with resonance states close to the Fermi level, which can be associated with the increased conductance. Finally, the translocation process of nucleobases passing through the nanogap was also investigated by varying their position from a certain height (from +3 to -3 angstrom) with respect to the graphene sheet to show that nitrogen-terminated sheets have enhanced sensitivity, as moving the nucleobase by approximately 1 angstrom reduces the conductance by up to 3 orders of magnitude.

  • 30.
    Amorim, Rodrigo G.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Silicene as a new potential DNA sequencing device2015In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 26, no 15, article id 154002Article in journal (Refereed)
    Abstract [en]

    Silicene, a hexagonal buckled 2D allotrope of silicon, shows potential as a platform for numerous new applications, and may allow for easier integration with existing silicon-based microelectronics than graphene. Here, we show that silicene could function as an electrical DNA sequencing device. We investigated the stability of this novel nano-bio system, its electronic properties and the pronounced effects on the transverse electronic transport, i.e., changes in the transmission and the conductance caused by adsorption of each nucleobase, explored by us through the non-equilibrium Green's function method. Intriguingly, despite the relatively weak interaction between nucleobases and silicene, significant changes in the transmittance at zero bias are predicted by us, in particular for the two nucleobases cytosine and guanine. Our findings suggest that silicene could be utilized as an integrated-circuit biosensor as part of a lab-on-a-chip device for DNA sequencing.

  • 31.
    Andersson, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Investigations of domain-wall motion using atomistic spin dynamics2015Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In this thesis, current driven domain-wall motion is studied using atomistic simulations with the exchange coupling modeled by the Heisenberg Hamiltonian under the nearest-neighbor approximation. The investigations may be divided into two parts, each concerned with how different aspects of the systems affect the domain-wall motion. The first part deals with domain-wall width dependence of the velocity in a three dimensional geometry with simple cubic crystal structure and uniaxial anisotropy. Results from this part showed that the velocity has a minor domain-wall width dependence. For a fixed current density, the velocity increased with domain-wall width, though only from 61.5 a/ns to 64.5 a/ns as the domain-wall width was increased from 3 to 25 atoms.

    The second part of the investigations deals with phenomena involving mixed cubic and uniaxial anisotropy, the non-adiabaticity parameter as well as the geometry of the system. The discussion includes an account of how the spin-transfer and cubic anisotropy torques contribute to the motion for different values of the non-adiabaticity parameter. In comparing a one dimensional atomic chain and a three dimensional system with simple cubic crystal structure, but otherwise with the same material properties, results showed a difference in how the two systems responded to currents. This difference is not accounted for by the micromagnetic theory, and its origin was unable to be determined.

  • 32.
    Anversa, Jonas
    et al.
    Univ Fed Santa Maria, Dept Fis, BR-97105900 Santa Maria, RS, Brazil.;Fac Meridional, Escola Engn Civil, BR-99070220 Passo Fundo, RS, Brazil..
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Piquini, Paulo
    Univ Fed Santa Maria, Dept Fis, BR-97105900 Santa Maria, RS, Brazil..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden..
    High pressure driven superconducting critical temperature tuning in Sb2Se3 topological insulator2016In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 108, no 21, article id 212601Article in journal (Refereed)
    Abstract [en]

    In this letter, we are reporting the change of superconducting critical temperature in Sb2Se3 topological insulator under the influence of an external hydrostatic pressure based on first principles electronic structure calculations coupled with Migdal-Eliashberg model. Experimentally, it was shown previously that Sb2Se3 was undergoing through a transition to a superconducting phase when subjected to a compressive pressure. Our results show that the critical temperature increases up to 6.15K under the pressure unto 40GPa and, subsequently, drops down until 70 GPa. Throughout this pressure range, the system is preserving the initial Pnma symmetry without any structural transformation. Our results suggest that the possible relevant mechanism behind the superconductivity in Sb2Se3 is primarily the electron-phonon coupling. Published by AIP Publishing.

  • 33.
    Aperis, Alex
    et al.
    Department of Physics, National Technical University of Athens, Athens, Greece.
    Kotetes, P.
    Varelogiannis, G.
    Oppeneer, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Small-q phonon-mediated unconventional superconductivity in the iron pnictides2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 9, p. 092505-Article in journal (Refereed)
    Abstract [en]

    We report self-consistent calculations of the gap symmetry for iron-based high-temperature superconductors using realistic small-q phonon-mediated pairing potentials and four-band energy dispersions. When both electron and hole Fermi surface pockets are present, we obtain the nodeless s± state that was first encountered in a spin-fluctuation mechanism picture. Nodal s± as well as other gap structures such as dx2−y2, s±+dx2−y2, and even a p-wave triplet state, are accessible upon doping within our phononic mechanism. Our results resolve the conflict between phase-sensitive experiments reporting a gap changing sign, attributed previously only to a nonphononic mechanism, and isotope effect measurements proving the involvement of phonons in the pairing.

  • 34.
    Aperis, Alex
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab initio theory of magnetic-field-induced odd-frequency two-band superconductivity in MgB22015In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 92, no 5, article id 054516Article in journal (Refereed)
  • 35.
    Aperis, Alex
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high-Tc superconductivity in FeSe/SrTiO32018In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Phys. Rev. B, Vol. 97, p. 060501(R)-Article in journal (Refereed)
  • 36.
    Arapan, Sergiu
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    High-pressure phase transformations in carbonates2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 18, p. 184115-Article in journal (Refereed)
    Abstract [en]

    High-pressure phase transitions sequences in CaCO3, SrCO3, and BaCO3 are studied by first-principle electronic structure calculations. Each of the carbonates undergoes the aragonite to postaragonite phase transition with pressure in agreement with the experimental observation of Ono et al. However, the postaragonite to post-postaragonite phase transition, predicted by Oganov et al. and later observed in CaCO3, is unlikely to occur in SrCO3 and BaCO3. Hence, the concept that isostructural compounds will exhibit the same type of pressure-induced phase transitions has limitations. A change of the hybridization of the carbon atom from sp(2) to sp(3) within the Pmcn phase occurs in each of compounds, thus the carbonates are likely to transform at very high pressure to structures with tetrahedral CO4-4 carbonate group.

  • 37. Arapan, Sergiu
    et al.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Volume-dependent electron localization in ceria2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, article id 125108Article in journal (Refereed)
    Abstract [en]

    We have performed a numerical study of the process of electron localization in reduced ceria. Our results show that different localized charge distributions can be attained in a bulk system by varying the lattice parameter. We demonstrate that the effect of electron localization is mainly determined by lattice relaxation and an accurate account for the effects of electronic correlation is necessary to achieve localized charge distribution.

  • 38.
    Araujo, Carlos Moyses
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nagar, Sandeep
    Ramzan, Muhammad
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Shukla, R.
    Jayakumar, O. D.
    Tyagi, A. K.
    Liu, Yi-Sheng
    Chen, Jeng-Lung
    Glans, Per-Anders
    Chang, Chinglin
    Blomqvist, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lizárraga, Raquel
    Holmstrom, Erik
    Belova, Lyubov
    Guo, Jinghua
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden.
    Rao, K. V.
    Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites2014In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, p. 4686-Article in journal (Refereed)
    Abstract [en]

    We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.

  • 39.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Appl Mat Phys, Dept Mat & Engn, S-10044 Stockholm, Sweden..
    Evaluating bulk Nb2O2F3 for Li-battery electrode applications2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 5, p. 3530-3535Article in journal (Refereed)
    Abstract [en]

    This investigation has the primary objective of elucidating the lithium intercalation process in the crystal structure of a new niobium oxyfluoride compound Nb2O2F3. The framework of the density functional theory was applied in a generalized gradient approximation together with the hybrid functional method. It is revealed that lithium atoms intercalate in this material in a maximum concentration of one Li atom per formula unit forming LiNb2O2F3. Moreover, octahedral positions in between the layers of Nb-O-F appear as the Li preferred occupancy resulting in a structural volume expansion of only 5%. Electronic structure evolution with the insertion of lithium displays a transformation from semi-conductor to metal when half of the lithium atoms are added. This transformation occurs due to a symmetry break induced by the transition from the + 8 to + 7 oxidation state of half of the Nb2 dimers. Then, after full lithiation the symmetry is recovered and the material becomes a semiconductor again with a band gap amounting to 1 eV. The evaluated average deintercalation potential reaches 1.29 V vs. Li/Li+ with activation energy for lithium ion migration of 0.79 eV. The computed low potential of the redox reaction Nb-2(8+) to Nb-2(7+) includes niobium oxyfluoride in the map of possible materials for the anode application of Li-ion batteries.

  • 40.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Banerjee, Amitava
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Panigrahi, Puspamitra
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Hindustan Univ, Ctr Clean Energy & Nanoconvergence, Madras, Tamil Nadu, India.
    Yang, Li
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Sjödin, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Strömme, Maria
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Araujo, C. Moyses
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat, Appl Mat Phys, S-10044 Stockholm, Sweden.; Royal Inst Technol KTH, Dept Engn, S-10044 Stockholm, Sweden.
    Assessing Electrochemical Properties of Polypyridine and Polythiophene for Prospective Application in Sustainable Organic Batteries2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 4, p. 3307-3314Article in journal (Refereed)
    Abstract [en]

    Conducting polymers are being considered promising candidates for sustainable organic batteries mainly due to their fast electron transport properties and high recyclability. In this work, key properties of polythiophene and polypyridine have been assessed through a combined theoretical and experimental study focusing on such applications. A theoretical protocol has been developed to calculate redox potentials in solution within the framework of the density functional theory and using continuous solvation models. Here, the evolution of the electrochemical properties of solvated oligomers as a function of the length of the chain is analyzed and then the polymer properties are estimated via linear regressions using ordinary least square. The predicted values were verified against our electrochemical experiments. This protocol can now be employed to screen a large database of compounds in order to identify organic electrodes with superior properties.

  • 41.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Banerjee, Amitava
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Panigrahi, Puspamitra
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Hindustan Univ, Ctr Clean Energy & Nanoconvergence, Chennai, Tamil Nadu, India.
    Yang, Li
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Strömme, Maria
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Sjödin, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Araujo, C. Moyses
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden.
    Designing strategies to tune reduction potential of organic molecules for sustainable high capacity batteries application2017In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 5, no 9, p. 4430-4454Article in journal (Refereed)
    Abstract [en]

    Organic compounds evolve as a promising alternative to the currently used inorganic materials in rechargeable batteries due to their low-cost, environmentally friendliness and flexibility. One of the strategies to reach acceptable energy densities and to deal with the high solubility of known organic compounds is to combine small redox active molecules, acting as capacity carrying centres, with conducting polymers. Following this strategy, it is important to achieve redox matching between the chosen molecule and the polymer backbone. Here, a synergetic approach combining theory and experiment has been employed to investigate this strategy. The framework of density functional theory connected with the reaction field method has been applied to predict the formal potential of 137 molecules and identify promising candidates for the referent application. The effects of including different ring types, e.g. fused rings or bonded rings, heteroatoms, [small pi] bonds, as well as carboxyl groups on the formal potential, has been rationalized. Finally, we have identified a number of molecules with acceptable theoretical capacities that show redox matching with thiophene-based conducting polymers which, hence, are suggested as pendent groups for the development of conducting redox polymer based electrode materials.

  • 42.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Unveiling the charge migration mechanism in Na2O2: implications for sodium-air batteries2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 12, p. 8203-8209Article in journal (Refereed)
    Abstract [en]

    Metal-air batteries have become promising candidates for modern energy storage due to their high theoretical energy density in comparison to other storage devices. The lower overpotential of Na compared with Li makes Na-air batteries more efficient in terms of battery lifetime. Additionally, the abundance of Na over Li is another advantage for Na batteries compared to Li batteries. Na2O2 is one of the main products of sodium-air battery reactions. The efficiency of air cells is always related to the charge transport mechanisms in the formed product. To unveil these diffusion mechanisms in one of the main products of the cell reaction Na-O-2 we systematically investigate the mobility of charge carriers as well as the electronic structural properties of sodium peroxide. The framework of the density functional theory based on hybrid functional approach is used to study the mobility of charge carriers and intrinsic defects in Na2O2. Our calculations reveal that the formation of small electron and hole polarons is preferentially occurring over the delocalized state in the crystal structure of Na2O2. The migration of these small polarons displays activation energies of about 0.92 eV and 0.32 eV for the electron and hole polarons respectively, while the analysis of the charged sodium vacancy mobility reveals an activation energy of about 0.5 eV. These results suggest that the charge transport in sodium peroxide would mainly occur through the diffusion of hole polarons.

  • 43.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Barpanda, Prabeer
    Indian Inst Sci, Faraday Mat Lab, Mat Res Ctr, CV Raman Ave, Bangalore 560012, Karnataka, India..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Appl Mat Phys, Dept Mat & Engn, S-10044 Stockholm, Sweden..
    Na2M2(SO4)(3) (M = Fe, Mn, Co and Ni): towards high-voltage sodium battery applications2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 14, p. 9658-9665Article in journal (Refereed)
    Abstract [en]

    Sodium-ion-based batteries have evolved as excellent alternatives to their lithium-ion-based counterparts due to the abundance, uniform geographical distribution and low price of Na resources. In the pursuit of sodium chemistry, recently the alluaudite framework Na2M2(SO4)(3) has been unveiled as a high-voltage sodium insertion system. In this context, the framework of density functional theory has been applied to systematically investigate the crystal structure evolution, density of states and charge transfer with sodium ions insertion, and the corresponding average redox potential, for Na2M2(SO4)(3) (M = Fe, Mn, Co and Ni). It is shown that full removal of sodium atoms from the Fe-based device is not a favorable process due to the 8% volume shrinkage. The imaginary frequencies obtained in the phonon dispersion also reflect this instability and the possible phase transition. This high volume change has not been observed in the cases of the Co- and Ni-based compounds. This is because the redox reaction assumes a different mechanism for each of the compounds investigated. For the polyanion with Fe, the removal of sodium ions induces a charge reorganization at the Fe centers. For the Mn case, the redox process induces a charge reorganization of the Mn centers with a small participation of the oxygen atoms. The Co and Ni compounds present a distinct trend with the redox reaction occurring with a strong participation of the oxygen sublattice, resulting in a very small volume change upon desodiation. Moreover, the average deintercalation potential for each of the compounds has been computed. The implications of our findings have been discussed both from the scientific perspective and in terms of technological aspects.

  • 44.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    de Almeida, J. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    da Silva, A. Ferreira
    Univ Fed Bahia, Inst Fis, Salvador, BA, Brazil..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Insights in the electronic structure and redox reaction energy in LiFePO4 battery material from an accurate Tran-Blaha modified Becke Johnson potential2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 12, article id 125107Article in journal (Refereed)
    Abstract [en]

    The main goals of this paper are to investigate the accuracy of the Tran-Blaha modified Becke Johnson (TB-mBJ) potential to predict the electronic structure of lithium iron phosphate and the related redox reaction energy with the lithium deintercalation process. The computed electronic structures show that the TB-mBJ method is able to partially localize Fe-3d electrons in LiFePO4 and FePO4 which usually is a problem for the generalized gradient approximation (GGA) due to the self interaction error. The energy band gap is also improved by the TB-mBJ calculations in comparison with the GGA results. It turned out, however, that the redox reaction energy evaluated by the TB-mBJ technique is not in good agreement with the measured one. It is speculated that this disagreement in the computed redox energy and the experimental value is due to the lack of a formal expression to evaluate the exchange and correlation energy. Therefore, the TB-mBJ is an efficient method to improve the prediction of the electronic structures coming form the standard GGA functional in LiFePO4 and FePO4. However, it does not appear to have the same efficiency for evaluating the redox reaction energies for the investigated system.

  • 45.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Islam, Muhammed Shafiqul
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Natl Univ Bangladesh, DSHE, Dhaka 1000, Bangladesh..
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden..
    Predicting electrochemical properties and ionic diffusion in Na2+2xMn2-x(SO4)(3): crafting a promising high voltage cathode material2016In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 4, no 2, p. 451-457Article in journal (Refereed)
    Abstract [en]

    Sodium ion batteries have emerged as a good alternative to lithium based systems due to their low cost of production. In this scenario, the search for higher voltage, sodium cathodes results in a new promising alluaudite structure Na2+2xMn2-x(SO4)(3). The structural, electronic and Na diffusion properties along with defects have been reported in this investigation within the framework of density functional theory. A band gap of 3.61 eV has been computed and the average deintercalation potential is determined to be 4.11 V vs. Na/Na+. A low concentration of anti-site defects is predicted due to their high formation energy. The biggest issue for the ionic diffusion in the Na2+2xMn2-x(SO4)(3) crystal structure is revealed to be the effect of Mn vacancies increasing the activation energy of Na+ ions that hop along the [001] equilibrium positions. This effect leads to activation energies of almost the same high values for the ionic hop through the [010] direction characterizing a 2D like ionic diffusion mechanism in this system.

  • 46.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    de Almeida, J. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ferreira da Silva, A.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lithium transport investigation in LixFeSiO4: A promising cathode material2013In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 173, p. 9-13Article in journal (Refereed)
    Abstract [en]

    In this paper we investigate lithium mobility in both Li2FeSiO4 and its   half-lithiated state LiFeSiO4 considering an orthorhombic crystal   structure. We find that the calculated activation energy of Li+ ions   hopping between adjacent equilibrium sites predicts two least hindered   diffusion pathways in both materials. One of them is along the [100]   direction characterizing an ionic diffusion in a straight line and the   other follows a zig-zag way between the Fe-Si-O layers. We also show   that diffusion of Li+ ions in the half-lithiated structure follows the   same behavior as in the lithiated structure. As a whole, the activation   energies for the investigated compounds present a greater value compared   with the activation energies in currently used materials such as   LiFePO4. The results were calculated in the framework of density   functional theory in conjunction with the climbing image nudged elastic   band method. The Hubbard term was added to the Kohn-Sham Hamiltonian to   overcome the delocalization problem of d electrons. Furthermore, the   diffusion coefficients were calculated for both structures considering   temperatures ranging from 300 to 700 K. (C) 2013 Elsevier Ltd. All   rights reserved.

  • 47.
    Araujo, Rafael Barros Neves de
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Banerjee, Amitava
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden..
    Divulging the Hidden Capacity and Sodiation Kinetics of NaxC6Cl4O2: A High Voltage Organic Cathode for Sodium Rechargeable Batteries2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 26, p. 14027-14036Article in journal (Refereed)
    Abstract [en]

    In the current emerging sustainable organic battery field, quinones are seen as one of the prime candidates for application in rechargeable battery electrodes. Recently, C6Cl4O2, a modified quinone, has been proposed as a high voltage organic cathode. However, the sodium insertion mechanism behind the cell reaction remained unclear due to the nescience of the right crystal structure. Here, the framework of the density functional theory (DFT) together with an evolutionary algorithm was employed to elucidate the crystal structures of the compounds NaxC6Cl4O2 (x = 0.5, 1.0, 1.5 and 2). Along with the usefulness of PBE functional to reflect the experimental potential, also the importance of the hybrid functional to divulge the hidden theoretical capacity is evaluated. We showed that the experimentally observed lower specific capacity is a result of the great stabilization of the intermediate phase Na1.5C6Cl4O2. The calculated activation barriers for the ionic hops are 0.68, 0.40, and 0.31 eV, respectively, for NaC6Cl4O2, Na1.5C6Cl4O2, and Na2C6Cl4O2. These results indicate that the kinetic process must not be a limiting factor upon Na insertion. Finally, the correct prediction of the specific capacity has confirmed that the theoretical strategy used, employing evolutionary simulations together with the hybrid functional framework, can rightly model the thermodynamic process in organic electrode compounds.

  • 48. Araujo, Rafael Barros Neves de
    et al.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    de Almeida, J. S.
    Ferreira da Silva, A.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    First-principles investigation of Li ion diffusion in Li2FeSiO42013In: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 247, p. 8-14Article in journal (Refereed)
    Abstract [en]

    We have studied the Li-ion migration and the electrochemical performance of Li2FeSiO4 in the monoclinic crystal structure with P2(1) symmetry and the related delithiated system LiFeSiO4. For this purpose, the framework of the density functional theory within the generalized gradient approximation in conjunction with the climbing image nudged elastic band method was used. Addition of the Hubbard term was also considered in the Kohn-Sham Hamiltonian to better model the d electrons of the metal ions in this material. The calculated activation energies for Li ion migration are found to decrease by around 20% with the Hubbard term inclusion in the chosen diffusion pathways of Li2FeSiO4. Regarding the delithiated structure, the activation energies were found to be sensitive to the Hubbard term addition, however no general behavior such as in the lithiated structure was found. Furthermore, the diffusion coefficients were calculated considering temperatures of 300 K, 500 K, and 700 K.

  • 49.
    Araújo, Rafael Barros Neves de
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Energy Storage Materials: Insights From ab Initio Theory: Diffusion, Structure, Thermodynamics and Design.2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The development of science and technology have provided a lifestyle completely dependent on energy consumption. Devices such as computers and mobile phones are good examples of how our daily life depends on electric energy. In this scenario, energy storage technologies emerge with strategic importance providing efficient ways to transport and commercialize the produced energy. Rechargeable batteries come as the most suitable alternative to fulfill the market demand due to their higher energy- and power- density when compared with other electrochemical energy storage systems. In this context, during the production of this thesis, promising compounds for advanced batteries application were investigated from the theoretical viewpoint. The framework of the density functional theory has been employed together with others theoretical tools to study properties such as ionic diffusion, redox potential, electronic structure and crystal structure prediction.

    Different organic materials were theoretically characterized with quite distinct objectives. For instance, a protocol able to predict the redox potential in solution of long oligomers were developed and tested against experimental measurements. Strategies such as anchoring of small active molecules on polymers backbone have also been investigated through a screening process that determined the most promising candidates. Methods such as evolutionary simulation and basin-hopping algorithm were employed to search for global minimum crystal structures of small molecules and inorganic compounds working as a cathode of advanced sodium batteries. The crystal structure evolution of C6Cl4O2 upon Na insertion was unveiled and the main reasons behind the lower specific capacity obtained in the experiment were clarified. Ab initio molecular dynamics and the nudged elastic band method were employed to understand the underlying ionic diffusion mechanisms in the recently proposed Alluaudite and Eldfellite cathode materials. Moreover, it was demonstrated that electronic conduction in Na2O2, a byproduct of the Na-O2 battery, occurs via hole polarons hopping. Important physical and chemical insights were obtained during the production of this thesis. It finally supports the development of low production cost, environmental friendliness and efficient electrode compounds for advanced secondary batteries. 

  • 50. Arhammar, C.
    et al.
    Silvearv, F.
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Norgren, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Pedersen, H.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    A theoretical study of possible point defects incorporated into alpha-alumina deposited by chemical vapor deposition2013In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 133, no 2, p. 1433-Article in journal (Refereed)
    Abstract [en]

    The energetics and electronic structure of carbon, chlorine, hydrogen, and sulfur in alpha-Al2O3 was investigated by first principles and thermodynamical calculations. These species are present in the gas phase during the synthesis of alpha-Al2O3 by chemical vapor deposition (CVD) but little is known of their solubility in this compound. The heat of formation from standard reference states of the elements varying the chemical potential of each element was calculated. An attempt to model the actual conditions in the CVD process was made, using the species and solid compounds present in a common CVD process as reference states. Our calculations suggest that sulfur from the catalyzing agent H2S will not solve in alpha-Al2O3 during deposition by CVD. It is found that the neutral chlorine and hydrogen interstitial defects display the lowest heat of formation, 281 and 280 kJ/mol, respectively, at the modeled CVD conditions. This energy is too high in order for neutral defects to form during CVD of alpha-Al2O3 at any significant amounts. The charged defects and their compensation were studied. Carbon substituting oxygen is found to be energetically favored under the modeled CVD conditions, considering carbon dioxide as competing species to solid solubility in alpha-Al2O3 at an energy of -128 kJ/mol. However, care needs to be taken when choosing the possible competing carbon-containing phases. Compensation of carbon substituting for oxygen by oxygen vacancies takes place at 110 kJ/mol from standard reference states, graphite, fcc-Al and O-2. The carbon solubility in Al2O3 is difficult to measure with standard analysis techniques such as X-ray diffraction and energy dispersive X-ray spectroscopy, but several stable compounds in the Al-C-O are available in the literature.

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