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Hydride Reduction of BaTiO3 - Oxyhydride Versus O Vacancy Formation.


ABSTRACT: 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 1H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products-indicative of the formation of BaTiO3-x H x -except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4-reduced samples) and 4.033 Å (for MgH2-reduced samples). With increasing H/BaTiO3 ratio, CaH2-, NaAlH4-, and MgH2-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-x H x with x values larger than 0.5. 1H MAS NMR spectroscopy, 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-x H y ?(x-y) with x up to 0.6 and y in a range 0.04-0.25, rather than homogeneous solid solutions BaTiO3-x H x . Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of negative chemical shifts and breadth of the signals signifies metallic conductivity and structural disorder in BaTiO3-x H y ?(x-y). Sintering of BaTiO3-x H y ?(x-y) in a gaseous H2 atmosphere resulted in more ordered materials, as indicated by considerably sharper 1H resonances.

SUBMITTER: Nedumkandathil R 

PROVIDER: S-EPMC6645482 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature

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We investigated the hydride reduction of tetragonal BaTiO<sub>3</sub> using the metal hydrides CaH<sub>2</sub>, NaH, MgH<sub>2</sub>, NaBH<sub>4</sub>, and NaAlH<sub>4</sub>. The reactions employed molar BaTiO<sub>3</sub>/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 <sup>1</sup>H magic angle spinning (MAS) NMR spectroscopy. PXRD sho  ...[more]

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