Pressure-induced behavior of the hydrogen-dominant compound ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$ from first-principles calculations

The structural and electronic properties of the high-pressure molecular compound ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$ have been calculated using density-functional theory. We identify the molecular hydrogen positions within the face-centered cubic unit cell and further find that pressure-induced intermolecular interaction between ${\text{SiH}}_4$ and ${\text{H}}_2$ units plays an important role in stabilizing this new compound. The electronic structure is characterized by a wide band gap of 6.1 eV at 6.8 GPa, which closes with pressure and finally becomes metallic at 200 GPa due to electronic band overlap accompanied by a structure change. These findings have potential implications for understanding metallization and superconductivity in ${\text{H}}_2$.

Figure 1

(Color online) Panel (a), Comparison of the calculated pressure-dependent volumes (equation of state) with available experimental data. Panel (b), calculated and experimental Raman spectra at 9 GPa and 8.6 GPa, respectively.

Figure 2

(Color online) Panels (a–c): the calculated total and local DOS at three pressures of 0 GPa, 6.8 GPa, and 200 GPa, respectively. ${\text{H}}_{\text{Si}}$ and ${\text{H}}_{\text{H}}$ denote the H atom from the units of ${\text{SiH}}_4$ and ${\text{H}}_2$ molecule, respectively. The Fermi level is set to zero. Panel (d): the calculated pressure-dependent charge increase, $q$, for the ${\text{H}}_2$ molecules.

Figure 3

(Color online) The crystal structure of ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$: panel (a) two-dimensional structural projection and panel (b) a three-dimensional structure. Large and small balls denote Si and H, respectively. Panel (c): contour plot of the charge difference in the (006) plane as marked in panel (b). Positive values denotes the charge accumulation and negative value charge depletion. ${}^{\ast }$ marks the region of charge accumulation in the interstitial spaces.

Figure 4

(Color online) Pressure-dependent band gap sizes (${\text{E}}_g$ in electron volt) by GGA-PBE approximations. Inset from left to right: the calculated electronic-band structures at 6.8 GPa, 199 GPa, and 200 GPa, respectively. Zero-energy lines indicate the top of valence band.