CRYSTAL-STRUCTURE OF HIGH-PRESSURE PHASE-IV SOLID HYDROGEN-SULFIDE

The crystal structure of high-pressure phase-IV solid hydrogen sulfide was studied by in situ angle-dispersive x-ray diffractometry using sy nchrotron radiation and an imaging plate. Analysis for the diffraction pattern obtained at 11.4 GPa shows that phase IV is monoclinic (space group Pc) as to the positions of sulfur atoms, which is structurally distorted from the cubic phases I and I' stable at low pressures. The volume reduction of the phase I'-to-IV transition is 2.3%. The variati on of the neighboring S-S distance in phase IV is different from phase s I and I', resulting in the disappearance of the orientationally diso rdered hydrogen bond.