Low-frequency Raman spectroscopy of deuterium to megabar pressures at 77-295 K

Rotational and lattice phonon excitations in the Raman spectrum of solid molecular deuterium have been measured from 1.8 GPa to {~}200 GPa at 77-295 K to study pressure-induced changes in structural and dynamical properties of the dense solid. Continuous and discontinuous changes in three distinct pressure ranges are observed. At lower pressures (30 GPa), there is a gradual increase in the linewidth of the S₀(0) band, together with gradually decreasing resolution of the higher-energy S₀(1) and S₀(2) features. At intermediate pressures (60--100 GPa), a change in the pressure dependence and linewidth of S₀(0), and a linewidth and intensity decrease in the E2g phonon band occur. This is interpreted as evidence for a phase transformation in the molecular solid beginning at {~}65 GPa (77 K). At the highest pressures ({~}160 GPa), abrupt changes in the low-frequency excitations suggest either an expansion of the molecular bond or a change in ordering at the vibron discontinuity. Evidence from the low-frequency spectra for interaction between the deuterium sample and diamond anvil is also examined.