Structural phase transition at high temperatures in solid molecular hydrogen and deuterium - art. no. 024108

We study the effect of temperature up to 1000 K on the structure of dense m olecular para-hydrogen (p-H-2) and ortho-deuterium (o-D-2). using the path- integral Monte Carlo method. We find a structural phase transition from ori entationally disordered hexagonal close packed (hcp) to an orthorhombic str ucture of Cmca symmetry before melting. The transition is basically induced by thermal fluctuations, but quantum fluctuations of protons (deuterons) a re important in determining the transition temperature through effectively hardening the intermolecular interaction. We estimate the phase line betwee n hcp and Cmca phases as well as the melting line of the Cmca solid.