Orientational disorder and the band gap of solid molecular hydrogen under pressure.

Abstract

The influence of librational motion (orientational disorder) on the electronic structure of molecular solid hydrogen is investigated by means of a microcrystal formalism. A cluster consisting of 64 molecules in the hexagonal close-packed structure with periodic boundary conditions is diagonalized exactly in a parametrized tight-binding approximation. 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 transitions occur just before the insulator-to-metal transition (closing of the gap). The theoretical results seem to explain satisfactorily the anomalies observed in dielectric measurements of solid hydrogen at high pressure.