THE SOLID-PHASES OF DEUTERIUM SULFIDE BY POWDER NEUTRON-DIFFRACTION

Powder neutron diffraction studies confirm that D2S has 3 solid-state phases at ambient pressure. Phases I and II are cubic and orientationally disordered with space groups Fm3m (a = 5.8486(8) angstrom at 160 K) and Pa3- (a = 5.7647(6) angstrom at 120 K), respectively. The deuterium scattering density arising from the disorder is modelled using symmetry-adapted spherical harmonic functions, which show a reduction from "twelve-fold" to "six-fold" disorder on cooling from phase I to phase II. For the fully ordered low-temperature phase III, a new structure has been determined which bears little resemblance to that reported in previous neutron diffraction study. The structure is orthorhombic, space group Pbcm, with a = 4.0760(1) angstrom, b = 13.3801(5) angstrom and c = 6.7215(3) angstrom at 1.5 K. The structure is essentially hexagonally close-packed, with distortions to accommodate the hydrogen atoms. The structure is antiferroelectric with the alignment of the molecular dipoles along the a direction.