New Phases and Dissociation-Recombination of Hydrogen Deuteride to 3.4 Mbar

We present infrared absorption studies of solid hydrogen deuteride to pressures as high as 340 GPa (100 GPa=1 Mbar) in a diamond anvil cell and temperatures in the range 5-295 K. Above 198 GPa the HD sample transforms to a mixture of HD, H_{2}, and D_{2}, interpreted as a process of dissociation and recombination. Three new phase lines are observed, two of which differ remarkably from those of the high-pressure homonuclear species, but none are metallic. The time-dependent spectral changes are analyzed to determine the molecular concentrations as a function of time; the nucleon exchange achieves steady state concentrations in ∼20 h at ∼200 GPa.