H-1 AND F-19 NUCLEAR MAGNETIC-RELAXATION STUDY OF THE DYNAMICS OF THE HYDROGEN DIFLUORIDE ION, FHF-, IN THE ALPHA-PHASE AND BETA-PHASE OF CESIUM HYDROGEN DIFLUORIDE

A H-1 and F-19 nuclear magnetic resonance spin-lattice relaxation investigation of polycrystalline caesium hydrogen difluoride, CsHF2, in the temperature range 175-390 K is reported. There is a discontinuous change in the relaxation behaviour in the region 330 +/- 2 K which corresponds to the structural phase transition. In the low temperature, or alpha-phase, the relaxation measurements are interpreted in terms of a model in which the FHF-ions undergo a 180-degrees-flip motion for which the activation energy is found to be 24.8 +/- 2.5 kJ mol-1. General relaxation expressions for a polycrystalline sample are developed for this motional model. A particular feature of the measurements is the cross-relaxation, or spin exchange, between the hydrogen and fluorine nuclei which results in non-exponential spin-lattice relaxation behaviour at temperatures below 200 K. In the cubic, or beta-phase, the relaxation measurements are shown to be consistent with a restricted rotational model in which reorientations of the FHF- ion occur at random among the three cubic unit cell directions. The activation energy for this type of motional process is found to be 19.6 +/- 1.5 kJ mol-1.