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Hydride Reduction of BaTiO3 – Oxyhydride vs O-Vacancy Formation
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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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.

Keyword [en]
Oxyhydrides, PXRD, NMR
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-141586OAI: oai:DiVA.org:su-141586DiVA: diva2:1087586
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-04-27Bibliographically approved
In thesis
1. Hydrogen incorporation in Zintl phases and transition metal oxides- new environments for the lightest element in solid state chemistry
Open this publication in new window or tab >>Hydrogen incorporation in Zintl phases and transition metal oxides- new environments for the lightest element in solid state chemistry
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. 86 p.
Keyword
Zintl phases, metal hydrides, transition metal oxyhydrides, XRPD, NPD, Rietveld refinement
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-141588 (URN)978-91-7649-789-0 (ISBN)978-91-7649-790-6 (ISBN)
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

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