Proton conductivity, proton transfer and hydroxyl ion reorientation were investigated, and expected H/D isotope effects upon their rates were compared to those of proton/deuteron conductivity. It was concluded that the rate of proton diffusivity in many oxides was limited by proton transfer between fixed O sites. This was aided by thermal fluctuations of the O ion separation. In the case of a rigid array of O ions, an attempt frequency which was close to that of an OH (OD) oscillator (together with a high activation enthalpy) was expected. It was found that the effect of a softening of the O-O vibration was to decrease the attempt frequency and activation barrier.

K.D.Kreuer, A.Fuchs, J.Maier: Solid State Ionics, 1995, 77, 157-62