Proton Diffusion Mechanism in Hydrated Barium Indate Oxides

Abstract

We report on quasielastic neutron scattering (QENS) and ab initio molecular dynamics (AIMD) simulations of the mechanism of proton diffusion in the partially and fully hydrated barium indate oxide proton conductors Ba2In2O5(H2O)x (x = 0.30 and 0.92). Structurally, these materials are featured by an intergrowth of cubic and “pseudo-cubic” layers of InO6 octahedra, wherein two distinct proton sites, H(1) and H(2), are present. We show that the main localized dynamics of these protons can be described as rotational diffusion of O-H(1) species and H(2) proton transfers between neighboring oxygen atoms. The mean residence times of both processes are in the order of picoseconds in the two studied materials. For the fully hydrated material, Ba2In2O5(H2O)0.92, we also reveal the presence of a third proton site, H(3), which becomes occupied upon increasing the temperature and serves as a saddle state for the interexchange between H(1) and H(2) protons. Crucially, the occupation of the H(3) site enables long-range diffusion of protons, which is highly anisotropic in nature and occurs through a two-dimensional pathway. For the partially hydrated material, Ba2In2O5(H2O)0.30, the occupation of the H(3) site and subsequent long-range diffusion are not observed, which is rationalized by hindered dynamics of H(2) protons in the vicinity of oxygen vacancies. A comparison to state-of-the-art proton-conducting oxides, such as barium zirconate-based materials, suggests that the generally lower proton conductivity in Ba2In2O5(H2O)x is due to a large occupation of the H(1) and H(2) sites, which, in turn, means that there are few sites available for proton diffusion. This insight suggests that the chemical substitution of indium by cations with higher oxidation states offers a novel route toward higher proton conductivity because it reduces the proton site occupancy while preserving an oxygen-vacancy-free structure.