On proton transport in perovskite-type oxides and plastic hydroxides

From a critical review of the experimental data on proton conducting oxides and new results of quantum MD-simulations on proton transport in the model perovskite-type oxide BaCeO3 further evidence is given for a rapid rotational diffusion of protonic defects (hydroxide ions on oxygen sites) with a comparatively slow proton exchange between neighbouring oxygens being the rate limiting step of long range proton transport. Proton transfer is suggested to occur almost barrierless in contracted transition state configurations of the type O-H-O which are virtually insensitive towards solvent effects. The energy of this configuration depends strongly on the softness of the oxide lattice, which is not only determined by the cation/oxygen bonding but also by the formation of hydrogen bonds of the type O-H ... O. Hydrogen bonding is expected to promote proton transport in oxides and the application of pressure, which generally increases the hydrogen bond strength, is anticipated to lower the activation energy for proton mobility. While protons in oxides may be approximately considered to be random walkers, proton transport in hydroxides is suggested to be highly correlated. For the cubic high temperature phases of alkaline metal hydroxides this is reflected by a high proton diffusion coefficients, while virtually no proton conductivity is observed. This effect is also expected to become relevant for proton transport in oxides with very high concentrations of protonic defects.