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Hydrogen incorporation in Zintl phases and transition metal oxides- new environments for the lightest element in solid state chemistry
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). (Ulrich Häussermann group)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This PhD thesis presents investigations of hydrogen incorporation in Zintl phases and transition metal oxides. Hydrogenous Zintl phases can serve as important model systems for fundamental studies of hydrogen-metal interactions, while at the same time hydrogen-induced chemical structure and physical property changes provide exciting prospects for materials science. Hydrogen incorporation in transition metal oxides leads to oxyhydride systems in which O and H together form an anionic substructure. The H species in transition metal oxides may be highly mobile, making these materials interesting precursors toward other mixed anion systems. 

Zintl phases consist of an active metal, M (alkali, alkaline earth or rare earth) and a more electronegative p-block metal or semimetal component, E (Al, Ga, Si, Ge, etc.). When Zintl phases react with hydrogen, they can either form polyanionic hydrides or interstitial hydrides, undergo full hydrogenations to complex hydrides, or oxidative decomposition to more E-rich Zintl phases. The Zintl phases investigated here comprised the CaSi2, Eu3Si4, ASi (A= K, Rb) and GdGa systems which were hydrogenated at various temperature, H2 pressure, and dwelling time conditions. For CaSi2, a regular phase transition from the conventional 6R to the rare 3R took place and no hydride formation was observed. In contrast, GdGa and Eu3Si4 were very susceptible to hydrogen uptake. Already at temperatures below 100 ºC the formation of hydrides GdGaH2-x and Eu3Si4H2+x was observed. The magnetic properties of the hydrides (antiferromagnetic) differ radically from that of the Zintl phase precursor (ferromagnetic). Upon hydrogenating ASi at temperatures around 100 oC, silanides ASiH3 formed which contain discrete complex ion units SiH3-. The much complicated β – α order-disorder phase transition in ASiH3 was evaluated with neutron powder diffraction (NPD), 2H NMR and heat capacity measurements. 

A systematic study of the hydride reduction of BaTiO3 leading to perovskite oxyhydrides BaTiO3-xHx was done. A broad range of reducing agents including NaH, MgH2, CaH2, LiAlH4 and NaBH4 was employed and temperature and dwelling conditions for hydride reduction examined. Samples were characterized by X-ray powder diffraction (XRPD), thermal gravimetric analysis and 1H NMR. The concentration of H that can be incorporated in BaTiO3-xHx was found to be very low, which is in contrast with earlier reports. Instead hydride reduction leads to a high concentration of O vacancies in the reduced BaTiO3. The highly O-deficient, disordered, phases - BaTiO3-xHy(x-y) with x up to 0.6 and y in a range 0.05 – 0.2 and (x-y) > y – are cubic and may represent interesting materials with respect to electron and ion transport as well as catalysis.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University , 2017. , p. 86
Keywords [en]
Zintl phases, metal hydrides, transition metal oxyhydrides, XRPD, NPD, Rietveld refinement
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-141588ISBN: 978-91-7649-789-0 (print)ISBN: 978-91-7649-790-6 (electronic)OAI: oai:DiVA.org:su-141588DiVA, id: diva2:1087588
Public defence
2017-05-29, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 5: Manuscript.

Available from: 2017-05-04 Created: 2017-04-07 Last updated: 2017-04-27Bibliographically approved
List of papers
1. The 3R polymorph of CaSi2
Open this publication in new window or tab >>The 3R polymorph of CaSi2
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2015 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 222, p. 18-24Article in journal (Refereed) Published
Abstract [en]

The Zintl phase CaSi2 commonly occurs in the 6R structure where puckered hexagon layers of Si atoms are stacked in an AA'BB'CC' fashion. In this study we show that sintering of CaSi2 in a hydrogen atmosphere (30 bar) at temperatures between 200 and 700 degrees C transforms 6R-CaSi2 quantitatively into 3R-CaSi2. In the 3R polymorph (space group R-3m (no. 166), a=3.8284(1), c=15.8966(4), Z=3) puckered hexagon layers are stacked in an ABC fashion. The volume per formula unit is about 3% larger compared to 6R-CaSi2. First principles density functional calculations reveal that 6R and 3R-CaSi2 are energetically degenerate at zero Kelvin. With increasing temperature 6R-CaSi2 stabilizes over 3R because of its higher entropy. This suggests that 3R-CaSi2 should revert to 6R at elevated temperatures, which however is not observed up to 800 degrees C. 3R-CaSi2 may be stabilized by small amounts of incorporated hydrogen and/or defects.

Keywords
Calcium disilicide, Zintl phases, Polymorphism, Structural stability
National Category
Chemical Sciences
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-114349 (URN)10.1016/j.jssc.2014.10.033 (DOI)000348633800004 ()
Note

AuthorCount:5;

Available from: 2015-03-09 Created: 2015-03-02 Last updated: 2017-04-27Bibliographically approved
2. Hydrogenation induced structure and property changes in GdGa
Open this publication in new window or tab >>Hydrogenation induced structure and property changes in GdGa
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2016 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 239, p. 184-191Article in journal (Refereed) Published
Abstract [en]

Hydrides GdGaH were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50 bar and temperatures from 50 to 500 degrees C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50 bar and temperatures below 500 degrees C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) angstrom, b=12.024(2) angstrom, c=4.1009(6) angstrom) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200 degrees C affords a hydride with x approximate to 1.3, which is stable up to 500 degrees C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200 degrees C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300 degrees C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of similar to 8 mu B which is typical for compounds containing Gd3+ ions.

Keywords
Metal hydrides, Zintl phases, in situ powder diffraction
National Category
Chemical Sciences
Research subject
Materials Chemistry; Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-131891 (URN)10.1016/j.jssc.2016.04.028 (DOI)000377422000027 ()
Available from: 2016-07-04 Created: 2016-07-04 Last updated: 2017-09-07Bibliographically approved
3. Hydrogen Induced Structure and Property Changes in Eu3Si4
Open this publication in new window or tab >>Hydrogen Induced Structure and Property Changes in Eu3Si4
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Hydrides Eu3Si4H2+x were obtained by exposing the Zintl phase Eu3Si4 to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 °C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25 – 200 ºC afford a uniform hydride phase with an orthorhombic structure (Immm, a ≈ 4.40 Å, b ≈ 3.97 Å, c ≈ 19.8 Å), whereas at 300 ºC mixtures of two orthorhombic phases with c ≈ 19.86 and ≈19.58 Å were obtained. The assignment of a composition Eu3Si4H2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H to be occupied in the Eu3Si4 structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu3Si4H2 is stable by -0.46 eV/H atom with respect to Eu3Si4 and gaseous H2. Deviations between the lattice parameters of the DFT optimized Eu3Si4H2 structure and the ones extracted from PXRD patterns point to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu3Si4H3 and Eu3Si4H4 showed considerably better agreement to the experimental unit cell volumes. However, the ordered monoclinic model structures do not provide a good match to the experimental, orthorhombic, PXRD patterns. It was then concluded that the hydrides of Eu3Si4 have a composition Eu3Si4H2+x (x < 2) and are disordered with respect to H in Si2Eu3 interstitials. Hydrides Eu3Si4H2+x decompose at temperatures above 300 °C in a dynamic vacuum into unidentified products. Thus the hydrogenation of Eu3Si4H2+x is not reversible. From magnetic measurements the Curie-Weiss constant and effective magnetic moment of Eu3Si4H2+x were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of ~8 mB which is typical for compounds containing Eu2+ 4f7 ions.  

Keywords
Zintl phases. metal hydrides
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-141585 (URN)
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-04-27Bibliographically approved
4. Investigation of the Order–Disorder Rotator Phase Transition in KSiH3 and RbSiH3
Open this publication in new window or tab >>Investigation of the Order–Disorder Rotator Phase Transition in KSiH3 and RbSiH3
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2017 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 9, p. 5241-5252Article in journal (Refereed) Published
Abstract [en]

The β–α (order–disorder) transition in the silanides ASiH3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (Cp), solid-state 2H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH3 moieties. At temperatures below 200 K ASiH3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH3 and KSiH3, respectively. The transition is accompanied by a large molar volume increase of about 14%. The Cp(T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH3 rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH3, the SiH3 anions undergo rotational diffusion with average relaxation times of 0.2–0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20–40 K). 2H NMR revealed that the α-form can coexist, presumably as 2–4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from Cp measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH3 near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH3 ions are only weakly coordinated in an environment of A+ cations. The orientational flexibility of SiH3 can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations.

Keywords
Zintl phases, Metal hydrides, Neutron powder diffraction
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-141587 (URN)10.1021/acs.jpcc.6b12902 (DOI)000396295800052 ()
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-05-02Bibliographically approved
5. Hydride Reduction of BaTiO3 – Oxyhydride vs O-Vacancy Formation
Open this publication in new window or tab >>Hydride Reduction of BaTiO3 – Oxyhydride vs O-Vacancy Formation
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4 and NaAlH4. The reactions employed molar BaTiO3:H ratios of up to 1.8 and temperatures near 600 °C. The air stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA) and solid-state 1H NMR spectroscopy. PXRD showed the formation of cubic products - indicative of the formation of BaTiO3-xHx - except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4 reduces samples) and 4.033 Å (for MgH2 reduced samples). With increasing BaTiO3:H ratio, CaH2, NaAlH4 and MgH2 reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increase of up to 3.5 % for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-xHx with x values larger 0.5. 1H NMR, however, revealed rather low concentrations of H, and, thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3-xHy(x-y) with x up to 0.6, y in a range 0.05 – 0.2 and (x-y) > y, rather than homogeneous solid solutions BaTiO3Hx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of chemical shifts and breadth of the signals signifies the structural disorder in BaTiO3-xHy(x-y). Sintering of BaTiO3-xHy(x-y) in a gaseous H2 atmosphere resulted in more ordered materials as indicated by considerably sharper 1H resonances.

Keywords
Oxyhydrides, PXRD, NMR
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-141586 (URN)
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-04-27Bibliographically approved

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