Megabar-pressure infrared study of hydrogen deuteride.

Solid hydrogen deuteride (HD) has been studied to a pressure of 159 GPa and to low temperatures using near infrared spectroscopy. Of the two high pressure phases observed in hydrogen and deuterium, known as the BSP (broken-symmetry phase) and the A phase, only the BSP had been observed in the lower pressure region of the phase line of HD and it was unusually different from the homonuclear diatomic species with a reentrant behavior. In this Letter the BSP phase line is identified to its maximum pressure of 159 GPa. Infrared absorption reveals a transition to the A phase, observed for the first time in HD with onset at 157+/-3 GPa. A new phase of electric dipolar order that should occur at low temperature is discussed.