  We present infrared absorption studies of solid hydrogen deuteride to
pressures as high as 3.4 megabar in a diamond anvil cell and temperatures in
the range 5 to 295 K. Above 198 GPa the sample transforms to a mixture of HD
,H2 and D2, interpreted as a process of dissociation and recombination. Three
new phases-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 hours at ~200 GPa.
