Neutron scattering investigation of the rotational dynamics and phase transitions in sodium and cesium hydrosulfides

Abstract

The rotational motions of the hydrosulfide ions in the trigonal and fcc phases of NaSH and in the pseudo‐bcc (CsCl) phase of CsSH have been studied by quasielastic and inelastic neutron scattering. NaSH and CsSH are members of a broad group of compounds M⁺(XY⁻) which have cubic symmetry in the solid phase just below the melting point and a lower symmetry in lower temperature crystal phases. The measured inelastic neutron spectra above and below the trigonal to cubic phase transition in NaSH show that SH⁻ ion ``librations'' about equilibrium orientations persist in passing through the transition. The maximum of the broad librational bands for both both NaSH and CsSH occurs near 400 cm⁻¹. A temperature and momentum‐transfer (Q) dependent broadening is observed, however, in the quasielastic peaks in the cubic phases of the hydrosulfides, which indicates a rapid reorientation of the SH⁻ ions. The experimental quasielastic scattering results are compared with theoretical calculations of quasielastic scattering behavior based on the assumption of instantaneous reorientational jumps between a limited number of quasiequilibrium orientations. The widths of the measured quasielastic peaks plotted vs Q show an oscillatory behavior as predicted by the theoretical calculations. An isotropic reorientation model is ruled out, and the differences in the rotational disorder in NaSH and CsSH are discussed. Relaxation times (τ) for the SH⁻ motions are derived from the theoretical analysis. The τ values for fcc NaSH vary from 0.4 to 0.15 psec between 103 and 212°C, while the values for pseudo‐bcc CsSH vary from 2.0 to 0.75 psec between 23 and 140°C.