ON PROTON TRANSPORT IN PEROVSKITE-TYPE OXIDES AND PLASTIC HYDROXIDES

From a critical review of the experimental data on proton conducting o xides 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 ox ygen sites) with a comparatively slow proton exchange between neighbou ring oxygens being the rate limiting step of long range proton transpo rt. Proton transfer is suggested to occur almost barrierless in contra cted transition state configurations of the type O-H-O which are virtu ally insensitive towards solvent effects. The energy of this configura tion depends strongly on the softness of the oxide lattice, which is n ot only determined by the cation/oxygen bonding but also by the format ion of hydrogen bonds of the type O-H ... O. Hydrogen bonding is expec ted to promote proton transport in oxides and the application of press ure, which generally increases the hydrogen bond strength, is anticipa ted 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 re flected by a high proton diffusion coefficients, while virtually no pr oton conductivity is observed. This effect is also expected to become relevant for proton transport in oxides with very high concentrations of protonic defects.