1H and19F nuclear magnetic relaxation study of the dynamics of the hydrogen difluoride ion, FHF–, in the α- and β-phases of caesium hydrogen difluoride

Abstract

A 1H and 19F 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 α–phase, the relaxation measurements are interpreted in terms of a model in which the FHF– ions undergo a 180 °-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 β-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.