Reactivity of Hydrogen-Helium and Hydrogen-Nitrogen Mixtures at High Pressures

Through a series of Raman spectroscopy studies, we investigate the behavior of hydrogen-helium and hydrogen-nitrogen mixtures at high pressure across a wide range of concentrations. We find that there is no evidence of chemical association or increased miscibility of hydrogen and helium in the solid state up to pressures of 250 GPa at 300 K. In contrast, we observe the formation of concentration-dependent ${\mathrm{N}}_2\text{−}{\mathrm{H}}_2$ van der Waals solids, which react to form N–H bonded compounds above 50 GPa. Through this combined study, we can demonstrate that the recently reported chemical association of ${\mathrm{H}}_2\text{−}\mathrm{He}$ can be attributed to significant ${\mathrm{N}}_2$ contamination and subsequent formation of ${\mathrm{N}}_2\text{−}{\mathrm{H}}_2$ compounds.

Figure 1

Raman frequency vs pressure for different ${\mathrm{H}}_2:\mathrm{He}$ concentrations up to 285 GPa. Circles and triangles represent data collected on compression and decompression, respectively. (Inset) Raman frequency vs pressure for different ${\mathrm{H}}_2:\mathrm{He}$ concentrations up to 20 GPa. Photomicrograph of a 10 mol% ${\mathrm{H}}_2$ in He sample at 15 GPa. The single crystal of ${\mathrm{H}}_2$, located in the center of the sample chamber, is clearly phase separated from the surrounding He medium.

Figure 2

(a) Evolution of the Raman spectra with composition in the solid state at around 40 GPa for the nitrogen-hydrogen compositions of 28, 59, and 75 mol % ${\mathrm{H}}_2$. The pure species are included for comparison in black. (b) Raman frequency vs pressure plots corresponding to the data shown in (a). Solid and dashed black lines correspond to pure ${\mathrm{H}}_2$ and pure ${\mathrm{N}}_2$, respectively. Arrows indicate weak Raman bands.

Figure 3

A comparison of the Raman frequencies as a function of pressure between our data on ${\mathrm{H}}_2\text{−}{\mathrm{N}}_2$ mixtures (dashed lines), pure ${\mathrm{H}}_2$ (black line), and the ${\mathrm{H}}_2\text{−}\mathrm{He}$ compound (known as “$S_2$”) reported in Ref. [20] (symbols). (Inset) Comparison between “H-He” vibrons from Ref. [20] and the ${\mathrm{N}}_2$ vibrons from a $3:2$ ${\mathrm{H}}_2\text{−}{\mathrm{N}}_2$ mixture.

Figure 4

Raman spectra on compression of a 50 mol% ${\mathrm{H}}_2$ composition demonstrating the time-dependent chemical reaction of ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ above 50 GPa. The blue shaded region indicates the formation of N–H bonds. The blue spectrum shows the recovery of hydrazine on decompression to 4 GPa. For a comparison between these data and Ref. [20], see Figs. S14 and S15 [36].