1H and 19F 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.