Ordered yttrium concentration effects on barium zirconate structure, proton binding sites and transition states

Abstract

Acceptor doped perovskites show promise as fuel cell proton conducting ceramic membranes and continue to be rapidly developed. Proton conduction in barium zirconate with 6.25% ordered yttrium doping at the zirconium site is explored. The dopant increases octahedral distortions in the lattice similar to our earlier studied 12.5% dopant system though the angle distribution is slightly broader. Normal modes promoting vibrations about a non-transient octahedral tilt are found for both concentrations at frequencies thermally accessible at typical proton conduction temperatures. The energy of proton binding sites and transition states roughly increases with distance away from the dopant though there are many oscillations in the trend due to local proton induced distortions. Protons can both locally enhance and decrease the octahedral distortions induced by the dopant. Transition state energies roughly increase with proton-dopant distance. Rotational and intraoctahedral transfer transition state energies vary through a similar magnitude range though the intraoctahedral transition states are often higher in energy. Interoctahedral transfer transition states tend to have the highest energies. The diversity of minima and transition states suggests that proton conduction in 6.25% yttrium doped barium zirconate is more complex than conduction in 12.5% doped barium zirconate.