We have studied solid molecular hydrogen at pressures up to 167 GPa (1.67 Mbar and temperatures from 4.3 to 150 K. We have investigated the phase transition recently observed by Hemley and Mao near 150 GPa and identified as orientational ordering by studying the Raman vibrational and rotational excitations. The pahse line determined for this transition differs significantly from that expected for the extension of the low-pressure orientational ordering. We conclude that this transition is to a new high-pressure molecular phase which only exists at megabar pressures.

• Received 22 June 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.2080