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 $157\pm 3\mathrm{GPa}$. A new phase of electric dipolar order that should occur at low temperature is discussed.

Figure 1

Phase diagram of HD, showing pressure-temperature points at which spectra were collected in the present experiment. The phase lines determined in this study are shown (solid lines), as well as the phase line determined earlier by Raman measurements (dashed lines) [6]. $A$ denotes the region where the $A$ phase is seen to be present. Also shown for comparison are the phase lines of ${\mathrm{D}}_2$ (dotted lines) [8]. The ${\mathrm{D}}_2$ BSP transition pressure measured in Ref. 8 was a bit lower than that measured in Ref. 5 due to the presence of a small concentration of para-$D$.

Figure 2

Absorption spectra for several pressures, collected at temperatures of 4–8 K. The arrows on the 36 GPa spectrum indicate double transitions; the up arrows on the 117 GPa spectrum indicate first-order vibron transitions, while the down arrow is a weak reproducible peak, possibly also a vibron transition. The up arrow on the 159 GPa spectrum indicates the appearance of a new vibron associated with the $A$ phase.

Figure 3

Absorption spectra collected at 159 GPa for several temperatures.