Hydride Reduction of BaTiO₃ - Oxyhydride Versus O Vacancy Formation

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 BaTiO_3- H -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 BaTiO_3- H 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 BaTiO_3- H □₍ with x up to 0.6 and y in a range 0.04-0.25, rather than homogeneous solid solutions BaTiO_3- 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_3- H □₍. Sintering of BaTiO_3- H □₍ in a gaseous H₂ atmosphere resulted in more ordered materials, as indicated by considerably sharper ¹H resonances.