Formation of ${\mathrm{H}}_2$-rich iodine-hydrogen compounds at high pressure

We have used synchrotron x-ray and Raman spectroscopic studies combined with molecular dynamics to investigate the ${\mathrm{I}}_2-{\mathrm{H}}_2$ system at high pressure. By laser heating the mixture above 25 GPa we synthesized the molecular compound $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$, with the ${AB}_{13}$ structure type and unusually high volumetric hydrogen content. The isolation of HI molecules by ${\left({\mathrm{H}}_2\right)}_{13}$ supramolecular clusters stabilizes the compound over a remarkable pressure range from 9 to at least 130 GPa. At lower pressures and 300 K another compound, hydrogen diiodane ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$, spontaneously forms, being stable up to 12.5 GPa.

Figure 1

X-ray diffraction structure analysis of $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$. Left: High-pressure x-ray diffraction pattern collected from a laser-heated mixture of ${\mathrm{H}}_2$ and ${\mathrm{I}}_2$ at 32.5 GPa ($\lambda =0.4141$ Å). Tick marks indicate Bragg peaks due to ${\mathrm{I}}_2$-II (top) and $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$ (bottom). The inset shows the same patterns with intensities scaled times 30 to highlight the peaks due to $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$. Middle: $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$ equation of state; symbols are experimental data from individual runs. The solid line corresponds to volume derived from the atomic equations of state of iodine and hydrogen. The inset shows x-ray diffraction patterns before and after laser heating. The well-defined rings at a low angle are due to $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$. Right: Changes in nearest and next-nearest $\mathrm{I}\cdots \mathrm{I}$ distances with pressure in HI (black), ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$ (dark blue), and $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$ (green). The low-pressure region is shown in the inset.

Figure 2

Representative Raman spectra and vibrational mode positions of $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$ and ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$. (a) Representative Raman spectra from a sample containing $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$ and pure ${\mathrm{H}}_2$ on compression and decompression. (b) Vibrational frequencies vs pressure of HI (light blue squares), ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$ (dark blue circles), $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$ (green diamonds), and ${\mathrm{H}}_2$ (black triangles) at 300 K. Solid symbols represent data on compression, while open symbols represent data collected on decompression. (c) Representative Raman spectra of the HI-${\mathrm{H}}_2-{\mathrm{I}}_2$ mixture (light blue), formation of ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$ at 3.9 GPa (dark blue), and its subsequent decomposition with pressure (black).

Figure 3

Molecular dynamic trajectories and structural model of $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$. (a) Time-averaged mean positions of selected atoms from the MD trajectory viewed along (110). The mean position of either atom in a rotating diatomic molecule is at the molecular center, even though the atom is never actually there. Consequently, the ${\mathrm{H}}_2$ molecules comprise two superimposed spheres, and only the iodine is visible for the HI molecules. (b) (${\mathrm{H}}_2{)}_{13}$ crystal structure with the orientation of icosahedral clusters highlighted with “bonds” between ${\mathrm{H}}_2$ positions to guide the eye.

Figure 4

X-ray diffraction structure analysis of ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$. Left: High-pressure x-ray diffraction pattern of ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$ at 7.1 GPa ($\lambda =0.4141$ Å), with the Le Bail profile refinement shown in red and difference shown in blue ($w{R}_p=2.39\%$). Tick marks indicate Bragg peaks due to ${\left(\mathrm{HI}\right)}_2{\mathrm{H}}_2$ and ${\mathrm{I}}_2$. Right: ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$ equation of state. The dashed line corresponds to the fitted equation of state, and the solid line indicates the volume derived from the corresponding atomic equations of state of ${\mathrm{I}}_2$ [11, 27] and ${\mathrm{H}}_2$ [28]. The volume per formula unit ($V/Z$) as a function of pressure shows good agreement with values calculated from the atomic equations of state for stoichiometry ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$, supporting the result of the refinement. The crystal structure of ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$ is body-centered-tetragonal with layers of HI molecules spaced along $c$ by the insertion of ${\mathrm{H}}_2$. HI molecules are represented by purple spheres; ${\mathrm{H}}_2$ molecules are shown by white spheres.