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  • 1.
    Biendicho, Jordi Jacas
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). STFC Rutherford Appleton Laboratory, England.
    Shafeie, Samrand
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Frenck, Louise
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Pierre & Marie Curie University, France.
    Gavrilova, Daria
    Böhme, Solveigh
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Chemnitz University of Technology, Germany.
    Bettanini, A. M.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Universita Ca Foscari Venezia, Italy.
    Svedlindh, Peter
    Hull, Steve
    Zhe, Zhao
    Istomin, S. Y.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Synthesis and characterization of perovskite-type SrxY1−xFeO3−δ (0.63≤x<1.0) and Sr0.75Y0.25Fe1−yMyO3−δ (M=Cr, Mn, Ni), (y=0.2, 0.33, 0.5)2013In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 200, p. 30-38Article in journal (Refereed)
    Abstract [en]

    Oxygen-deficient ferrates with the cubic perovskite structure SrxY1-xFeO3-d were prepared in air (0.71 ≤ x ≤ 0.91) as well as in N2 (x=0.75 and 0.79) at 1573K. The oxygen content of the compounds prepared in air increases with increasing strontium content from 3-δ=2.79(2) for x=0.75 to 3-δ=2.83(2) for x=0.91. Refinement of the crystal structure of Sr0.75Y0.25FeO2.79 using TOF neutron powder diffraction (NPD) data shows high anisotropic atomic displacement parameter (ADP) for the oxygen atom resulting from a substantial cation and anion disorder. Electron diffraction (ED) and high-resolution electron microscopy (HREM) studies of Sr0.75Y0.25FeO2.79 reveal a modulation along <100>p with G± ~0.4<100>p indicating a local ordering of oxygen vacancies. Magnetic susceptibility measurements at 5-390K show spin-glass behaviour with dominating antiferromagnetic coupling between the magnetic moments of Fe cations. Among the studied compositions, Sr0.75Y0.25FeO2.79 shows the lowest thermal expansion coefficient (TEC) of 10.5 ppm K-1 in air at 298-673K. At 773-1173K TEC increases up to 17.2 ppm K-1 due to substantial reduction of oxygen content. The latter also results in a dramatic decrease of the electrical conductivity in air above 673K. Partial substitution of Fe by Cr, Mn and Ni according to the formula Sr0.75Y0.25Fe1-yMyO3-δ (y=0.2, 0.33, 0.5) leads to cubic perovskites for all substituents with y=0.2. Their TECs are higher in comparison with un-doped Sr0.75Y0.25FeO2.79. Only M=Ni has increased electrical conductivity compared to un-doped Sr0.75Y0.25FeO2.79.

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    fulltext
  • 2.
    Shafeie, Samrand
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Properties in New Complex Perovskite-Related Materials, a Matter of Composition and Structure2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This PhD thesis presents investigations of perovskite-related compounds in systems of interest for applications in components in solid oxide fuel cells. The compound compositions derive from substitutions in the parent compounds LaCoO3, LaCrO3 and SrFeO3.

    Novel phases La2Co1+z(MgxTi1-x)1-zO6 were synthesized and investigated with regard to structure, thermal expansion, electronic and magnetic properties. The study focused on the composition lines La2Co(MgxTi1-x)O6 (z=0), where the oxidation state of Co nominally changes from +2 (x=0.0) to +3 (x=0.5), and La2Co1+z(Mg0.5Ti0.5)1-zO6, with a varying fraction of Co3+ ions. XANES data show that the Co ions in the system have discrete oxidation states of +2 and +3. The TEC increases with increasing x due to an increasing contribution from spin state transitions of the Co3+ ions. Novel compounds La2Cr(M2/3Nb1/3)O6 with M=Mg, Ni, Cu were synthesized and characterized with respect to structure and magnetic properties. XRPD and NPD data indicate Pbnm symmetry; however, SAED patterns and HREM images indicate a P21/n symmetry for M=Mg, and Cu. The magnetic measurements results were rationalized using the Goodenough-Kanamori rules.

    Oxygen-deficient phases with x≥0.63 in SrxY1-xFeO3-δ and Sr0.75Y0.25Fe1-yMyO3-δ (M=Cr, Mn, Ni and y=0.2, 0.33, 0.5), were synthesized and characterized with respect to structure, oxygen content, thermogravimetry, TEC, conductivity and magnetic properties. Powder patterns of phases agree with cubic  perovskite structures. NPD data for x=0.75 reveal anisotropic displacement for the O atom, related to local effects from Fe3+/Fe4+ ions. SAED patterns for x=0.75 reveal the presence of an incommensurate modulation. The compounds start to lose oxygen in air at ~ 400°C. The TEC up to ~400°C for x=0.75 is ~10.5 ppm/K and increase to ~17.5 ppm/K at higher temperatures. The conductivity for x=0.91 is 164 S/cm at 400°C. Partial substitution of Fe by Cr, Mn or Ni does not increase the conductivity or decrease TEC.

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    Fulltext_PhD_thesis_Samrand_Shafeie
  • 3.
    Shafeie, Samrand
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Structure and Properties Investigations of the La2Co1+z(Ti1-xMgx)1-zO6 Perovskite System2011Licentiate thesis, monograph (Other academic)
    Abstract [en]

    Perovskite based materials have great potentials for various energy applications and the search for new materials for uses in SOFCs has largely been concentrated to this class of compounds. In this search, we have studied perovskite phases in the system La2Co1+z(Ti1-xMgx)1-zO6, with 0  x 0.9 and z = 0.0, 0.2, 0.4, 0.6. Crystal structures were characterized by XRD and, for selected compositions, also by NPD and SAED. They exhibit with increasing x, as well as increasing z, a progressive increase in symmetry from monoclinic to orthorhombic to rhombohedral. The main focus in this work has been on the investigation of structure-property relations for compositions with 0.0 x 0.5 and z = 0. The nominal oxidation state of Co increases for these with increasing x, from Co2+ for x = 0 to Co3+ for x = 0.5. Magnetic measurements and XANES studies showed that the average spin state of Co changes linearly with increasing x, up to x = 0.5, in accordance with varying proportions of Co with two fixed oxidation states, i.e. Co2+ and Co3+. The data suggests that the Co3+ ions have an IS spin state or a mixture of LS and HS spin states for all compositions with nominally only Co2+ and Co3+ ions, possibly with the exception of the composition with x = 0.1, 0.2 and z = 0, for which the data indicate that the spin state might be HS. The XANES data indicate furthermore that for the perovskite phases with z = 0 and x > 0.5, which in the absence of O atom vacancies contain formally Co4+, the highest oxidation state of Co is Co3+, implying that the substitution of Ti4+ by Mg2+ for x ³ 0.5 effects an oxidation of O2- ions rather than an oxidation of Co3+ ions.

    The thermal expansion was found to increase nearly linearly with increasing oxidation state of Co. This agrees well with findings in previous studies and is attributable to an increase in the ionic radius of Co3+ ions with increasing temperature, due to a thermal excitation from a LS to IS or LS/HS spin states. High temperature electronic conductivity measurements indicate that the electronic conductivity increases with an increase of both relative and absolute amount of Co3+. The latter can be attributed to an increase in the number of Co-O-Co connections. Additional high temperature magnetic measurements for selected samples, whose susceptibilities did not follow a Curie law behaviour up to room temperature, showed effective magnetic moments that did approach plateaus even at high temperatures (900 K). Interpretations of these data are, however, hindered by the samples losing oxygen during the applied heating-cooling cycle.

    The present study has shown that the investigated system is suitable for further studies, of more fundamental character, which could provide further insight of the structure-property relationships that depend on the oxidation state of Co.

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    FULLTEXT02
  • 4.
    Shafeie, Samrand
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Istomin, S. Y.
    Karvonen, L.
    Chen, S. A.
    Chen, T. H.
    Chen, J. M.
    Weidenkaff, A.
    Karppinen, M.
    Sirtl, T.
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1-x)1-zO62011In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 184, no 1, p. 177-190Article in journal (Refereed)
    Abstract [en]

    Perovskite-type cobaltates in the system La(2)Co(1+z) (Mg(x)Ti(1-x))(1-z)O(6) were studied for z=0 <= x <= 0.6 and 0 <= x <= 0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption near-edge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350 degrees C. The space group symmetry of the structure changes from P2(1)/n via Pbnm to R (3) over barc with both increasing Mg content and increasing Co content. The La(2)Co(Mg(x)Ti(1-x))O(6) (z=0) compounds show anti-ferromagnetic couplings of the magnetic moments for the Co below 15 K for x=0, 0.1 and 0.2. XANES spectra show for the compositions 0 <= x <= 0.5 a linear decrease in the L(3)/(L(3)+ L(2))Co-L(2.3) edge branching ratio with x, in agreement with a decrease of the average Co ion spin-state, from a high-spin to a lower-spin-state, with decreasing nominal Co(2+) ion content.

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    FULLTEXT01
  • 5.
    Shafeie, Samrand
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Istomin, S. Ya
    Gippius, A. A.
    Karvonen, L.
    Populoh, S.
    Weidenkaff, A.
    Koehler, J.
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tracking of high-temperature thermal expansion and transport properties vs. oxidation state of cobalt between +2 and +3 in the La2Co1+z(Ti1-xMgx)(1-z)O-6-system2012In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 32, p. 16269-16276Article in journal (Refereed)
    Abstract [en]

    The high-temperature thermal expansion and electronic transport properties of the B-site substituted LaCoO3 with both variable oxidation state of cobalt between +2 and +3 (La2Co(Ti1-xMgx)O-6, 0 <= x <= 0.5) and variable Co3+-content relative to the other B-cations (La2Co1+z(Ti0.5Mg0.5)(1-z)O-6, 0.2 <= z <= 0.6) have been investigated. Based on the temperature dependence of the thermal expansion, electronic transport properties and Seebeck coefficient, three different groups of compositions according to their symmetries can be allocated. It was found that the thermal expansion coefficients (TECs) of the studied compounds are mainly dependent on the proportion of Co2+/Co3+. For La2Co(Ti1-xMgx)O-6, the TEC increases from similar to 9 (x = 0) to similar to 19 ppm K-1 (x = 0.5) with an increase of the oxidation state of cobalt from +2 to +3, respectively. The TECs of La2Co1+z(Ti0.5Mg0.5)(1-z)O-6, z = 0.2-0.6 with Co3+-only, remain constant at similar to 21 ppm K-1 independent of the cobalt content. Thermoelectric measurements of the system indicate that all samples in the system, except La2Co1.6(Ti0.5Mg0.5)(0.4)O-6, are p-type conductors over the whole temperature range, 300 < T < 1175 K. The conductivities were found to increase with an increase of both Co3+ and total cobalt content and are described with a small polaron hopping model. Due to an insignificant number of oxygen vacancies of La2Co1+z(Ti1-xMgx)(1-z)O-6 samples prepared in air at elevated temperatures, the investigated system is proposed as an excellent model system for the investigation of the influence of the Co oxidation state and stoichiometry on different properties in perovskite cobalt oxides.

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    La2CoMgTiO6_HT_fin
  • 6.
    Svensson, Gunnar
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shafeie, Samrand
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Masson, D.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Norberg, S. T.
    Eriksson, S.
    Hull, S.
    Zakharov, K. V.
    Volkova, O. S.
    Vasil'ev, A. N.
    Istomin, S. Ya.
    Synthesis and characterisation of the novel double perovskites La2CrB2/3Nb1/3O6, B = Mg, Ni, Cu2012In: Materials research bulletin, ISSN 0025-5408, E-ISSN 1873-4227, Vol. 47, no 9, p. 2449-2454Article in journal (Refereed)
    Abstract [en]

    The novel perovskites La2CrB2/3Nb1/3O6, B = Mg, Ni, and Cu have been synthesised at 1350 degrees C in air via the citrate route. Rietveld refinements using neutron powder diffraction (NPD) data showed that the compounds adopt the GdFeO3 type structure with space group Pbnm, and unit cell parameters a approximate to b approximate to root 2 x a(p) and c approximate to 2 x a(p), where a(p) approximate to 3.8 angstrom. Selected area electron diffraction (SAED) of B = Ni and Cu samples confirmed space group Pbnm. However, distinct reflections forbidden in Pbnm symmetry, but allowed in the monoclinic sub-group P2(1)/n and unit cell parameters a approximate to b approximate to root 2 x a(p) and c approximate to 2 x a(p), beta approximate to 90 degrees were present in SAED patterns of B = Mg sample. This indicates an ordering of the B-cations within the crystal structure of La2CrMg2/3Nb1/3O6. High-resolution electron microscopy (HREM) study indicating uniform, without formation of clusters, ordering of B-cations in the crystallites of La2CrMg2/3Nb1/3O6. Magnetic susceptibility measurements show that the compounds are antiferromagnetic (with some glass or spin clustering effects due to additional ferromagnetic interactions between the B-cations) with T-N for La2CrB2/3Nb1/3O6, B = Mg, Ni, Cu being 90, 125 and 140K, respectively.

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