Density-Dependent Phase Transition and Anisotropic Interactions in Solid Parahydrogen and Orthodeuterium

Raman scattering studies of the $J=0\mathrm{}\to 2$ rotational transitions in solid parahydrogen and orthodeuterium over the pressure range $0<~p<~3.3$ kbar have revealed a phase transition from hcp to fcc, with the fcc structure being stable at high pressures and hcp stable at low pressures (≲ 0.8 kbar). In both phases the main anisotropic interaction is molecular electric quadrupole-quadrupole ($\sim R^{-5\mathrm{}}$), but we observe significant effects tentatively attributed to correlated zero-point motion.