Melting and High $P\text{−}T$ Transitions of Hydrogen up to 300 GPa

High $P\text{−}T$ Raman spectra of hydrogen in the vibron and lattice mode regions were measured up to 300 GPa and 900 K using externally heated diamond anvil cell techniques. A new melting line determined from the disappearance of lattice mode excitations was measured directly for the first time above 140 GPa. The results differ from theoretical predictions and extrapolations from lower pressure melting relations. In addition, discontinuities in Raman frequencies are observed as a function of pressure and temperature indicative of phase transition at these conditions. The appearance of a new Raman feature near $2700{\mathrm{cm}}^{-1}$ at $\sim 300\mathrm{GPa}$ and 370 K indicates the transformation to a new crystalline phase. Theoretical calculations of the spectrum suggest the new phase is the proposed $Cmca\text{−}4$ metallic phase. The transition pressure is close to that of a recently reported transition observed on dynamic compression.

Figure 1

Pressure-temperature paths for different heating runs. The small pressure variations were recorded by diamond Raman spectra collected at each temperature increment (see Ref. [38]). Solid and open symbols indicate $P\text{−}T$ regions where solid and melt, respectively, were observed; pink dashed line, Bonev et al. [10]; blue dashed line, Morales et al. [12]; red dotted line, Caillabet et al. [13]; green dot-dashed line, Belonoshko et al. [14]; red dashed line, Liu et al. [15]. The inset shows examples of diamond anvil Raman spectra for measurement of pressure during heating.

Figure 2

Selected Raman spectra of hydrogen at selected temperatures and pressures. Top: Low-frequency region. The flattened spectra (red thick dashed lines) show the disappearance of lattice modes on melting. The black thin dashed lines show the behavior of individual modes during heating. Bottom: High-frequency vibron region. Spectra in red thick dashed lines are those obtained at melting; the labels ${\nu }_1$ and ${\nu }_2$ represent the vibrons from the two-layer structure of phase IV. The asterisk indicates the new peak that appears above $\sim 280\mathrm{GPa}$ on heating.

Figure 3

Hydrogen $P\text{−}T$ phase diagram. The data below 140 GPa have been fit with a Kechin equation: $T_m(K)=14.025{(1+P/a)}^b\exp (-P/c)$, with $a=0.029683$, $b=0.60224$, $c=116.4$. Red solid circles and black solid lines are the melting points and the fitted melting lines from this study. The two black dashed lines at $\sim 140$ and $\sim 230\mathrm{GPa}$ represent proposed phase lines in the solid. Previous experimental data: downwards open triangles, Diatschenko et al. [33]; crosses, Datchi et al. [5]; open squares, Gregoryanz et al. [6]; diamond, Deemyad and Silvera [7]; upwards open triangles, Eremets and Trojan [8]; open circles, Subramanian et al. [36]; solid upwards triangles (150 GPa, absorption onset; 300 GPa, reflectivity onset), Knudson et al. [23]; solid downwards triangle, Zaghoo et al. [26]. Previous proposed phase lines: red solid line, Zha et al. [51]; light gray short-dashed lines, Howie et al.[35]; blue dotted line, Bonev et al. [10]; red dotted line, Caillabet et al. [13]; gray long-dashed line and green dashed lines, Morales et al. [12]; red dashed line, Lorenzen et al. [28]; pink short-dashed line, Pierleoni et al. [32]; green dot-dashed line, Belonoshko et al. [14].