Mechanism of Hydride-Ion Diffusion in the Oxyhydride of Barium Titanate: Insights from Quasielastic Neutron Scattering

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Perovskite type oxyhydrides, of the form ATiO3-xHx (A = Ba, Sr, and Ca, and x < 0.6) are a novel class of mixed-anion materials displaying high hydride-ion conductivity. The high hydride-ion conductivity may be exploited for applications such as in energy storage and conversion devices, and in catalyst technologies for hydrogenation reactions, bu the conductivity mechanism is not fully understood. Here, in a quasielastic neutron scattering study of the oxyhydride of barium titanate, with different levels of anion vacancies, we show that the rate of hydride-ion diffusion increases with an increasing level of anion vacancies, and that the presence of anion vacancies is a necessity for hydride-ion conductivity. The conductivity mechanism is highly temperature dependent. Below 350 K, the hydride ions undergo jumps amongst nearest neighbor anion vacancies acting as jumping sites, whereas at temperatures between 350 and 550 K the hydride ions also undergo jumps amongst second nearest neighbor sites in the material. The jump diffusion dynamics are characterized by an average relaxation time on the order of 10 ps with a diffusion coefficient of the order of 10-5 cm2/s. For temperatures above 300 K, an additional, slower timescale process, on the order of 100 ps, which may be related to nearest neighbour hydride-ion jumps in different types of local structures with different anion and vacancy concentrations and distributions is observed. Tailoring of the local structure with respect to the concentration and distribution of anion vacancies may thus be a promising route towards optimizing the hydride-ion conductivity towards specific applications