Neutron Diffraction Study of the Trigonal and Cubic Phases of NaSH

Abstract

Neutron diffraction powder patterns have been measured for the trigonal and cubic phases of NaSH at 296 and 379°K, respectively. Several possible models describing the orientations of the SH^– ion in the trigonal phase were compared with the experimental results. A model in which the SH^– ions are aligned along the trigonal axis gave the best agreement between calculated and observed structure factors. Thus the transition from the cubic to trigonal phase apparently results from the alignment of the SH^– ions along one of the cube diagonals which then becomes shorter than the others resulting in the less symmetric trigonal structure. Several structural models were also considered for the cubic phase. The isotropic, free rotator mode describing the SH^– orientations can be rejected in favor of a less randomly disordered model by comparison with preliminary neutron inelastic scattering data. The limited diffraction data in this phase, however, do not enable a distinction to be made between disordered models. The preliminary inelastic scattering results also indicate that the librational frequencies of the SH^– ions are similar in both phases, but that reorientations of the SH^– ion between equivalent directions occur at a much faster rate ${\text{(∼10}}^{12}\hspace{0.17em}{\sec }^{\text{−1}})$ in the cubic phase.