PROTON DIFFUSION MECHANISM IN 3 SOLID-PHASES OF CSOH.H2O

The results of high precision PFG-NMR measurements of the self-diffusion coefficient, D(s), in the three upper solid phases (MT, LHT and UHT) of CsOH.H2O are presented and analyzed together with high-resolution QINS spectra of the same substance. In the temperature range studied, 349 K less-than-or-equal-to T less-than-or-equal-to 4.7 K, D(s) varies by the factor 20, up to about 1.5 x 10(-7) cm2/s. The variation can be described by Arrhenius laws with activation energies of 3130 K (MT phase), 8402 K (LHT phase) and 5779 K (UHT phase). By an analysis of the EISF due to a local diffusive proton motion in the UHT phase and by studying the Q-dependent quasielastic linewidth caused by long-range translational diffusion of protons, we show that the rate-determining process for the latter is the exchange of protons between the two minima of the H-bond double-well potential. The (much faster) local motion turns out to consist of 120-degrees-reorientational jumps of OH- groups around the c-axis of the hexagonal lattice.