Hydride Reduction of BaTiO - Oxyhydride Versus O Vacancy Formation.

We investigated the hydride reduction of tetragonal BaTiO using the metal hydrides CaH, NaH, MgH, NaBH, and NaAlH. The reactions employed molar BaTiO/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 H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products-indicative of the formation of BaTiO H -except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH-reduced samples) and 4.033 Å (for MgH-reduced samples). With increasing H/BaTiO ratio, CaH-, NaAlH-, and MgH-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO, suggesting that metal hydride reduction yielded oxyhydrides BaTiO H with values larger than 0.5. H MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO. It has to be concluded that hydride reduction of BaTiO yields complex disordered materials BaTiO H □ with up to 0.6 and in a range 0.04-0.25, rather than homogeneous solid solutions BaTiO H . 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 BaTiO H □. Sintering of BaTiO H □ in a gaseous H atmosphere resulted in more ordered materials, as indicated by considerably sharper H resonances.