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

Raman scattering studies of the $J=0\ensuremath{\rightarrow}2$ 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 (\ensuremath{\lesssim} 0.8 kbar). In both phases the main anisotropic interaction is molecular electric quadrupole-quadrupole ($\ensuremath{\sim}{R}^{\ensuremath{-}5}$), but we observe significant effects tentatively attributed to correlated zero-point motion.

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