Neutron quasielastic scattering study of SH− reorientation in the cubic phases of cesium and rubidium hydrosulfide

The orientational disorder of the hydrosulfide ions in CsSH (CsCl phase) and RbSH (NaCl phase) has been investigated by quasielastic neutron scattering with high energy resolution ${\mathrm{(\Delta \; E}}_{\text{1/2}\max }\text{=0.25\hspace{0.17em}}\mathrm{meV})$ . The experimental results provide a clear demonstration of the theoretically predicted separation of the quasielastic neutron peaks for rotating groups or molecules into unbroadened and broadened components which reflect, respectively, the geometric and time behavior of the rotation. Jump reorientation of the ions between equilibrium directions is established as the dominant mechanism creating the rotational disorder in the hydrosulfides, and both small‐step rotational diffusion and quasifree rotation are clearly ruled out. Average residence times between reorientation jumps are derived from comparisons of the experimental results with theoretical calculations based on jump reorientation models, but it is not possible to determine the equilibrium orientation of the SH⁻ ions. Mean‐square vibrational amplitudes for the hydrogen atoms are also obtained from the observed integrated intensities of the elastic peaks. The present results are compared in detail with previous lower‐resolution neutron results on NaSH and CsSH. It is concluded that in most cases measurements using single crystals will be necessary to establish the details of orientation disorder in solids.