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.

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The β−α (order−disorder) transition in the silanides ASiH 3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (C p ), solid-state 2 H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH 3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH 3 − moieties. At temperatures below 200 K ASiH 3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH 3 and KSiH 3 , respectively. The transition is accompanied by a large molar volume increase of about 14%. The C p (T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH 3 − rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH 3 , the SiH 3 − anions undergo rotational diffusion with average relaxation times of 0.2−0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20−40 K). 2 H NMR revealed that the α-form can coexist, presumably as 2−4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from C p measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH 3 near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH 3 − ions are only weakly coordinated in an environment of A + cations. The orientational flexibility of SiH 3 − can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations.

Perovskite-type oxyhydrides, BaTiO3–x H x , have been recently shown to exhibit hydride-ion (H–) conductivity at elevated temperatures, but the underlying mechanism of hydride-ion conduction and how it depends on temperature and oxygen vacancy concentration remains unclear. Here, we investigate, through the use of quasielastic neutron scattering techniques, the nature of the hydride-ion dynamics in three metal hydride-reduced BaTiO3 samples that are characterized by the simultaneous presence of hydride ions and oxygen vacancies. Measurements of elastic fixed window scans upon heating reveal the presence of quasielastic scattering due to hydride-ion dynamics for temperatures above ca. 200 K. Analyses of quasielastic spectra measured at low (225 and 250 K) and high (400–700 K) temperature show that the dynamics can be adequately described by established models of jump diffusion. At low temperature, ≤250 K, all of the models feature a characteristic jump distance of about 2.8 Å, thus of the order of the distance between neighboring oxygen atoms or oxygen vacancies of the perovskite lattice and a mean residence time between successive jumps of the order of 0.1 ns. At higher temperatures, >400 K, the jump distance increases to about 4 Å, thus of the order of the distance between next-nearest neighboring oxygen atoms or oxygen vacancies, with a mean residence time of the order of picoseconds. A diffusion constant D was computed from the data measured at low and high temperatures, respectively, and takes on values of about 0.4 × 10–6 cm–2 s–1 at the lowest applied temperature of 225 K and between ca. 20 × 10–6 and 100 × 10–6 cm–2 s–1 at temperatures between 400 and 700 K. Activation energies E a were derived from the measurements at high temperatures and take on values of about 0.1 eV and show a slight increase with increasing oxygen vacancy concentration.

Different forms of reduced BaTiO3, which include oxyhydride BaTiO2.9H0.1 and O-deficient BaTiO2.9−xH0.1□x, were obtained from reactions with LiH at various temperatures.