Abstract
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.