Proton Diffusion in Proton-Conducting Ba₂In_1.85M_0.15O₅(M = In, Ga, Sc, and Y) Investigated with Quasielastic Neutron Scattering

Hydrated barium indate oxide, Ba₂In₂O₅(H₂O), is an ideal model system for intermediate-temperature proton-conducting oxides based on the brownmillerite structure. However, fundamental questions surrounding the defect chemistry, proton dynamics, and how they depend on temperature and cation substitution in this material remain to be understood. Here, we investigate the nature of diffusional proton dynamics in Ba₂In₂O₅(H₂O)_xand the cation-substituted materials Ba₂In_1.85M_0.15O₅(H₂O)_x(M = Ga, Sc, and Y) with x = 0.70–0.92, using quasielastic neutron scattering (QENS). Analysis of the QENS data measured upon heating from 22 to 600 K reveals the onset of diffusional proton dynamics at approximately 400 K for the cation-substituted materials, Ba₂In_1.85M_0.15O₅(H₂O)_x(M = Ga, Sc, and Y), whereas no proton dynamics are observed for the pristine material Ba₂In₂O₅(H₂O)_x, in the here probed time-range of 30–300 ps. For temperatures between 400 and 490 K, the proton dynamics can be assigned to a mixture of rotational diffusion of O–H species and proton transfers between neighboring oxygens, with mean residence times in the range of 103–730 ps for both processes, quite independent of M. At the highest measured temperature, 600 K, the observed proton dynamics are of long-range character and can be approximated with a Chudley–Elliott model of jump diffusion with a mean jump length and mean residence time between two successive jumps of approximately 2.5 Å and 0.12 ns, respectively. Our results indicate that substituting indium with trivalent cations, to increase the availability of accessible proton sites along the diffusion pathway, promotes long-range proton diffusion to occur on a faster time scale in cation-substituted barium indate than in the pristine material.