A Theory of Crystal Distortion under High Pressure in Solid Hydrogen

Abstract

A mechanism of crystal distortion under high pressure is discussed in solid normal hydrogen. The strong interaction between the lattice vibration and the degenerate rotational states on orthohydrogens is treated in a form similar to the cooperative Jahn-Teller effect. The crystal field due to the local distortion can be very large under high pressure, leading to continuous decrease of the axial ratio, c⁄a, from its ideal value of the hcp structure with increasing pressure at room temperature, in agreement with the recent experiments. Isotope effects are also discussed. The first order transition of the distortion is predicted as a function of temperature.