Structure and Phase Transition in Solid Hydrogen and Deuterium Sulphides

Abstract

SPECIFIC heat measurements^1–3 have shown that solid hydrogen sulphide undergoes a discontinuous transition at 103.5 K followed by a lambda transition which is completed at about 126.2 K. Similar transitions were found⁴ in solid deuterium sulphide at somewhat higher temperatures, 107.8 and 132.9 K respectively. The lowest solid phase of both substances is optically anisotropic⁴ whereas the two higher phases are isotropic. Dielectric constant measurements^5–7 suggested that the orientation of the hydrogen sulphide molecules is ordered in the lowest solid phase and disordered in the two higher phases. This view gained further support from infrared^8–11, Raman^12,13 and nuclear magnetic resonance^14,15 studies. No model has been put forward for the structure of the ordered phase; the investigators could not agree even on its symmetry. Up to now eight different point group symmetries have been proposed, the majority favouring tetragonal symmetry with eight molecules in the primitive unit cell.