High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures

In situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium demonstrate the presence of a novel phase, phase ${\mathrm{II}}^{\prime }$, unique to deuterium and distinct from the known phase II. Phase ${\mathrm{II}}^{\prime }$ of ${\mathrm{D}}_2$ is not observed in hydrogen, making it the only phase that does not exist in both isotopes and occupies a significant part of $P\text{−}T$ space from $\sim 25$ to 110 GPa and below 125 K. For ${\mathrm{H}}_2$, the data show that below 30 K the transition to phase II happens at as low as 73 GPa. The transformation from phase II to III commences at around $\sim 155\mathrm{GPa}$ and is completed by 170 GPa with the average pressure of $\sim 160\mathrm{GPa}$ being slightly higher than previously thought. The updated phase diagrams of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ demonstrate the difference between the isotopes at low temperatures and moderate pressures, providing new information on the phase diagrams of both elements.

Figure 1

Representative Raman spectra at different $P\text{−}T$ conditions. (a) Representative spectra of ${\mathrm{H}}_2$ as a function of pressure at 20 K. (b) Representative spectra of ${\mathrm{H}}_2$ as a function of temperature at 92 GPa. (c) Representative spectra of ${\mathrm{D}}_2$ as a function of pressure at 40 K. (c) Representative spectra of ${\mathrm{D}}_2$ as a function of temperature at 82 GPa. The different colors of the curves represent different phases: green, I; blue, ${\mathrm{II}}^{\prime }$; maroon, II.

Figure 2

(a) The representative Raman spectra of the vibrational excitation of ${\mathrm{D}}_2$ as a function of temperature. The vibrational excitation split and shift happen during the phase I to ${\mathrm{II}}^{\prime }$ transition. The Raman frequency of ${\mathrm{H}}_2$ (b) and ${\mathrm{D}}_2$ (c), (d) as a function of temperature at various pressures. The solid circles are the experimental data, the dashed lines are only a visual aid. The error bars are only shown on one curve for clarity.

Figure 3

Proposed phase diagrams of hydrogen (a) and deuterium (b) in a low-temperature, medium-pressure range. Different colors represent different phases in accordance with Fig. 1. The gray dashed lines are the phase boundaries from Ref. [26]. The darker shaded area around the phase transition lines indicates the area where the two phases coexist. The dashed-dotted line in (b) between 20 and 40 GPa along the I-II phase line indicates the area previously proposed as phase ${\mathrm{II}}^{\prime }$ [8].