Hydride Reduction of BaTiO(3) − Oxyhydride Versus O Vacancy Formation

[Image: see text] We investigated the hydride reduction of tetragonal BaTiO(3) using the metal hydrides CaH(2), NaH, MgH(2), NaBH(4), and NaAlH(4). The reactions employed molar BaTiO(3)/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 (1)H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products—indicative of the formation of BaTiO(3–x)H(x)—except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH(4)-reduced samples) and 4.033 Å (for MgH(2)-reduced samples). With increasing H/BaTiO(3) ratio, CaH(2)-, NaAlH(4)-, and MgH(2)-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO(3), suggesting that metal hydride reduction yielded oxyhydrides BaTiO(3–x)H(x) with x values larger than 0.5. (1)H MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO(3). It has to be concluded that hydride reduction of BaTiO(3) yields complex disordered materials BaTiO(3–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 BaTiO(3–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 BaTiO(3–x)H(y)□((x–y)). Sintering of BaTiO(3–x)H(y)□((x–y)) in a gaseous H(2) atmosphere resulted in more ordered materials, as indicated by considerably sharper (1)H resonances.