Atomic structure and ${\text{SiH}}_4{\text{-H}}_2$ interactions of ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$ from first principles

First-principles density-functional theory (DFT) calculations are used to understand the crystal structure, bonding, and vibrational properties of the recently discovered high-pressure ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$ compound. We find a general decrease in the frequencies of the intramolecular ${\text{H}}_2$ stretching modes with increasing pressure, where the tetrahedral ${\text{H}}_2$ exhibit markedly stronger softening than octahedral ${\text{H}}_2$. Our DFT results suggest a weakening of the ${\text{H}}_2$ bond that is explained by increased orbital overlap and electron sharing between the silane and hydrogen molecules, which also account for the unusually high hydrogen capacity of ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$.

Figure 1

(Color online) Proposed conventional cell of ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$. Pink (light) spheres represent hydrogen atoms in ${\text{H}}_2$. Blue (dark) spheres represent silicon atoms and the surrounding tetrahedra show the orientation of the ${\text{SiH}}_4$ groups where hydrogen atoms are positioned at the vertices.

Figure 2

(Color online) (a) Calculated ${\text{H}}_2$ vibrational frequencies in the $T=0\text{K}$ structure of ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$ as a function of pressure. Points on the continuous lines give the average frequencies of ${\text{H}}_2$ in their respective sites. Bars show the range of frequencies for the two sites. (b) Phonon density of states for ${\text{H}}_2$ in ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$ at $T=300\text{K}$.

Figure 3

(Color online) Calculated electronic band structure of ${\text{SiH}}_4{\left({\text{H}}_2\right)}_2$ at a pressure of 5.6 GPa (with $\Gamma$ to X being perpendicular to ${\text{H}}_2$ orientations) and the electronic density of states at pressures of 5.6 and 23.9 GPa (Ref. 19).