The crystal structure and SiH$_{4}$-H$_{2}$ interactions of high-pressure SiH$_{4}$(H$_{2})_{2}$ from first principles

Mixtures of SiH$_{4}$ and H$_{2}$ have recently been found to crystallize at pressures above 6.8 GPa. Here, the crystal structure, bonding, and vibrational properties of SiH$_{4}$(H$_{2})_{2}$ over a range of applied pressures are studied using first-principles density functional theory (DFT) calculations. Results show a decrease in the frequencies of the intramolecular H$_{2}$ stretching modes with increasing pressure, contrary to the behavior of bulk H$_{2}$ under an applied pressure. This softening of the H$_{2}$ bond is found to be much more prominent for H$_{2}$ located in tetrahedral sites rather than in octahedral sites. DFT calculations suggest that the behavior of the H$_{2}$ bond is explained by an increased orbital overlap and electron sharing between silane and hydrogen molecules.