Hydrosulfide-ion dynamics in cesium and rubidium hydrosulfide: a deuteron nuclear magnetic resonance study

Abstract

RbSD and CsSD have a multiplicity of solid-state phase transitions involving changes in the degree of order of the SD− ion. Because the deuteron has a nuclear quadrupole moment, the observed NMR spectrum reflects any changes that take place in the deuteron-site symmetry as a result of a phase change. Furthermore, the magnitude of the observed nuclear quadrupole interaction depends on the time average of the electric-field gradient at the deuteron site; this, in general, is a function of any molecular motion in the crystal. The nuclear spin–lattice relaxation times provide information about the time scale of any molecular reorientation taking place in the crystal structure. Deuteron NMR spectra and relaxation times are presented for RbSD and CsSD over the temperature range from 100 to 400 K. The spin–lattice relaxation time data show that there is reorientation of the SD− ion in the tetragonal phase of CsSD and in the trigonal phase of RbSD. While the correlation time for the reorientation changes from being short compared with the reciprocal of the quadrupole interaction to being the same order of magnitude in the temperature range studied, the deuterium NMR line shapes do not change substantially. It is concluded that the observed reorientation of the SD− ion in both RbSD and CsSD in the low-temperature noncubic phases is end-for-end flipping of the SD− ion since only reorientation by 180° leaves the static quadrupole splitting unchanged.