Infrared study of high-pressure HD: observation of the A-phase

Infrared absorption was used to investigate the phase diagram of solid HD up to pressures of 156 GPa at temperatures ranging from 4 to 200K. A re-entrant phase line between the low-pressure and broken-symmetry phases (BSP) was observed, in agreement with Raman scattering results, with a 0 K transition pressure of $\sim $ 65-70 GPa. A phase transition was observed with an onset at $\sim $154 GPa (at 5K), consistent with the transition to the A-phase, previously observed in H$_{2}$ and D$_{2}$. The infrared spectra in the compressed low-pressure and BSP phases complement existing Raman spectra in these phases.