New Phases and Dissociation-Recombination of Hydrogen Deuteride to 3.4 Mbar

We present infrared absorption studies of solid hydrogen deuteride to pressures as high as 340 GPa ($100\mathrm{GPa}=1\mathrm{Mbar}$) in a diamond anvil cell and temperatures in the range 5–295 K. Above 198 GPa the HD sample transforms to a mixture of HD, ${\mathrm{H}}_2$, and ${\mathrm{D}}_2$, interpreted as a process of dissociation and recombination. Three new phase 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 $\sim 20\mathrm{h}$ at $\sim 200\mathrm{GPa}$.

Figure 1

The phase diagram of HD, extending the earlier measurements beyond the triple point to establish the I–III line. For comparison, we show the phase diagram of ${\mathrm{D}}_2$. Solid lines (above $\sim 200\mathrm{GPa}$) indicate new phase lines of mixtures of isotopes. For the III to $\mathrm{HD}\text{−}\mathrm{IV}*$ line, we only traversed the phase line at $P\sim 198\mathrm{GPa}$, $T=82$, and 240 K, indicated by open stars, while DISREC was observed all along the phase line (filled stars). The shaded area shows where hydrogen, deuterium, and hydrogen deuteride spectra were observed. We also show a DISREC point at room temperature and a point from Howie et al. [12] where they observed a phase transition.

Figure 2

(a) Infrared spectra as a function of pressure at a temperature of 82 K. At 146 GPa the impurity hydrogen vibron line is too weak to be observed in phase II. With increasing pressure one sees the onset of deuterium lines and growth of hydrogen lines, evidence of DISREC, as well as splitting of the spectral lines. For the spectrum at 198 GPa, the sample was in the process of transformation and not at final equilibrium concentrations. Right: IR spectra as a function of temperature for the indicated pressures. (b) As temperature is lowered, a transition from PRE to ${\mathrm{IV}}^{*}$ is observed; (c) shows the reversal, ${\mathrm{IV}}^{*}$ to PRE.

Figure 3

Peak frequencies of the IR modes as a function of pressure, indicating phase transitions where splittings or new modes appear. The shaded area is where a mixed isotope sample, due to DISREC, was observed.

Figure 4

The normalized IR integrated intensity changes of hydrogen isotopes plotted as a function of time, showing the kinetics associated with the DISREC at 198 GPa and 82 K. These time-dependent intensity changes are fitted to Avrami’s equation (see the inset) to yield the rate constants and Avrami exponent marked on the plots.