Phase Diagram of Dense ${\mathrm{H}}_2\text{−}\mathrm{He}$ Mixtures: Evidence for Strong Chemical Association, Miscibility, and Structural Change

The phase diagram of hydrogen-helium mixtures is presented to 75 GPa, underscoring the formation of metastable ${\mathrm{H}}_2$-rich crystallite in He-rich fluid mixtures and the structural phase transition in He lattice at $\sim 52\mathrm{GPa}$. The Raman data also indicate a significant level of mixing between ${\mathrm{H}}_2$ and He even in solids, giving rise to new vibrational bands in He-rich solid at $\sim 2400{\mathrm{cm}}^{-1}$ for H-He stretching and $140{\mathrm{cm}}^{-1}$ for the lattice phonon of ${\mathrm{H}}_2$ incorporated hcp He. Therefore, the present result signifies unexpected, strong chemical association of the interstitial-filled guest molecules (${\mathrm{H}}_2$ or He) with the host lattice (hcp He or ${\mathrm{H}}_2$) in this quantum solid mixture.

Figure 1

Binary phase diagram of ${\mathrm{H}}_2+\mathrm{He}$ mixtures to 75 GPa, signifying the phase transition of He-rich solid $S_2$ to $S_3$ at 52 GPa and a formation of metastable ${\mathrm{H}}_2$-rich solid $S_1^{\prime }$ in He-rich mixtures. Closed (open) symbols with different colors denote compression (decompression) data for different He compositions as used for the rest of the figures. $F$, $F_1$, and $F_2$ refer respectively to homogenous ${\mathrm{H}}_2\text{−}\mathrm{He}$ fluid mixture, ${\mathrm{H}}_2$-rich fluid, and He-rich fluid, whereas $S_1$ and $S_1^{\prime }$ are ${\mathrm{H}}_2$-rich solids and $S_2$ and $S_3$ refer to He-rich solids.

Figure 2

Raman spectral changes of ${\mathrm{H}}_2$ vibrons in $1:1$ ${\mathrm{H}}_2:\mathrm{He}$ mixture along the 300 K isothermal compression to 75 GPa. $F$, $F_1$, $F_2$, $S_1$, and $S_2$ signify a homogeneous fluid mixture, ${\mathrm{H}}_2$-rich fluid, He-rich fluid, ${\mathrm{H}}_2$-rich solid, and He-rich solid, respectively, $S_3$ also refers to He-rich solid above 52 GPa. Their Raman spectra are plotted in different colors.

Figure 3

Raman spectral change of $1:1$ ${\mathrm{H}}_2:\mathrm{He}$ solid mixture at high pressures, plotted in the low-frequency regions of (a) lattice phonon at $100–300{\mathrm{cm}}^{-1}$ and (b) H-He vibron at $2340–2440{\mathrm{cm}}^{-1}$. (c) The H-He vibron at 20 GPa is also plotted as a function of He concentration, for comparison. The inset shows the peak intensity at $2370{\mathrm{cm}}^{-1}$, normalized to that of the second-order diamond Raman at $2570{\mathrm{cm}}^{-1}$, as a function of He concentration. The $S_2$ and $S_3$ signify He-rich solids at pressures below and above 52 GPa, respectively.

Figure 4

Pressure-dependent Raman shift of ${\mathrm{H}}_2$ vibron in ${\mathrm{H}}_2+\mathrm{He}$ mixtures at various concentrations: [(a) and (b)] in the high-frequency region (a) below and (b) above 12 GPa, and (c) in the low-frequency region to 75 GPa. The inset in (a) represents a microphotograph of $1:9$ ${\mathrm{H}}_2:\mathrm{He}$ mixture at 12.6 GPa, showing coexistence of three different solid phases, $S_1$, $S_2$ and $S_1^{\prime }$, and those of (b) and (c) refer to the ${\mathrm{H}}_2$ vibrons of $S_1^{\prime }$ at 28 GPa of the same mixture.