Orientational phase transitions in hydrogen at megabar pressures.

Abstract

We have studied solid molecular hydrogen in various ortho-para concentrations at megabar pressures and down to liquid-helium temperatures. From changes of Raman spectra of rotational and vibrational modes we have identified three new phases. We show evidence for the orientationally ordered phase of parahydrogen at a pressure of 110 GPa (1.1 Mbar) and 8 K and for molecular orientational ordering within the newly discovered hydrogen-A phase.