The effect of cation substitution on the local coordination of protons in Ba2In1.85M0.15O6H2 (M = In, Ga, Sc and Y)

We report on an investigation of the local structure and vibrational dynamics in the brownmillerite-based proton conductors Ba2In1.85M0.15O6H2 with M = In, Ga, Sc and Y. The aim is to determine the effect of the cation (M) substitution on the local coordination environment of the protons. The techniques used are infrared spectroscopy and inelastic neutron scattering. The materials are characterized by two main types of proton sites, denoted as H(1) and H(2), which are featured by different local structures. We establish that the relative population of these two proton sites varies as a function of M. Specifically, it is found that, with respect to Ba2In2O6H2, the relative population of H(1) protons increases upon the substitution of In with any of the three different cations. The strongest effect is observed for M = Ga and Sc, whereas the effect observed for M = Y is minor. This new insight motivates efforts to unravel the mobility of the two types of protons, since then cation modification would offer a rational route for improving the proton conductivity of these types of materials.