ORIENTATIONAL DISORDER AND THE BAND-GAP OF SOLID MOLECULAR-HYDROGEN UNDER PRESSURE

The influence of librational motion (orientational disorder) on the el ectronic structure of molecular solid hydrogen is investigated by mean s of a microcrystal formalism. A cluster consisting of 64 molecules in the hexagonal close-packed structure with periodic boundary condition s is diagonalized exactly in a parametrized tight-binding approximatio n. The influence of pressure and libration are examined for a variety of orientational structures. It is found that the stable orientational structure varies with pressure, and that orientational phase transiti ons occur just before the insulator-to-metal transition (closing of th e gap). The theoretical results seem to explain satisfactorily the ano malies observed in dielectric measurements of solid hydrogen at high p ressure.