Excitations, order parameters, and phase diagram of solid deuterium at megabar pressures

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Abstract
In recent infrared experiments on solid ortho-deuterium a number of IR-active modes were observed in the various high-pressure phases. We have performed a group-theoretical analysis of IR and Raman activity for the vibrational modes of proposed space groups to aid in determining the structures of the various phases. From a comparison of the theoretical analysis with experimental results, candidate structures for the various phases are selected. The best candidates have underlying molecular centers which form hcp lattices in both the broken-symmetry phase (BSP) and the deuterium-A (D-A) phases; the molecules in each phase are orientationally ordered along crystalline directions not coinciding with the c axis. The hcp-c structure has been excluded. We have also analyzed the mode frequencies and integrated intensities in these phases to determine the order parameters for the structures. From the temperature dependence of the order parameters a determination of the order of the phase transition can be made. The D-A phase does not end in a critical point, but rather a triple point with a phase line separating the low pressure phase and the D-A phase, above the triple point. Our results are consistent with a first-order phase transition in crossing from the BSP to the D-A phase and a second-order phase transition from the D-A phase to the low pressure orientationally disordered phase.