H/D ISOTOPE EFFECT OF PROTON CONDUCTIVITY AND PROTON CONDUCTION MECHANISM IN OXIDES

The elementary steps of proton conductivity in oxides, proton transfer and hydroxyl ion reorientation, are investigated and the anticipated H/D isotope effects of their rates are compared to those of proton (deuteron) conductivity. it is concluded that the rate of proton diffusivity in many oxides is limited by proton transfer between fixed oxygen sites. This is facilitated by the thermal fluctuation of the oxygen ion separation. In the case of a rigid array of oxygen ions an attempt frequency close to that of the OH (OD) oscillator and a high activation enthalpy is expected. The effect of softening the O-O vibration is to decrease the attempt frequency and activation barrier. Criteria for high diffusivities of protonic defects in oxides are formulated.