It was noted that measurements of the dependence of the ionic conductivity upon temperature and hydrostatic pressure permitted the estimation of the volume relaxations which were associated with proton diffusion. Activation volumes were deduced from impedance spectra that were measured at pressures of up to 30kbar, and temperatures ranging from ambient to 400C. It was found that, at temperatures above 200C and pressures below 5kbar, the activation energies and pre-exponential factors were very high. In this region, the activation volume was quite large, negative, and exhibited a marked dependence upon temperature. Nuclear magnetic resonance measurements at normal pressures showed that, at temperatures above 180C, this resonance peak width was so small that translational motion of NH4+/NH3 must were occurring; even if this was not confirmed by nuclear scattering data. The activation volumes which were calculated here supported the operation of a vehicle conduction model within the studied temperature/pressure region. At pressures higher than 8kbar, the activation volume was positive over the entire temperature range. There was a small decrease with temperature (2.8 to 1.8cm3/mol) which was explained in terms of thermal expansion of the lattice; which led to a lower resistance to transport, and less deformation. These activation volumes were of the same order as those (2.8cm3/mol) reported for NASICON. Although this volume was high for proton transport, only a Grotthuss mechanism seemed to be occurring. The fact that the activation enthalpies were different at high and low temperatures was attributed to a greater efficiency of ammonium ions in Grotthuss transport at higher temperatures. Thermal vibrations of the N-H bonds were expected to increase the effectiveness of proton transfer to the host structure, or to a neighboring NH3, by temporarily decreasing the distance to an alternative site for the proton.

R.Hinrichs, G.Tomandl, J.A.H.Da Jornada: Solid State Ionics, 1995, 77, 257-62