Hydride Reduction of BaTiO3 − Oxyhydride Versus O Vacancy Formation

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.