Atomic structure and SiH4-H2 interactions of SiH4(H2)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 SiH4(H-2)(2) compound. We find a general decrease in the frequencies of the intramolecular H-2 stretching modes with increasing pressure, where the tetrahedral H-2 exhibit markedly stronger softening than octahedral H-2. Our DFT results suggest a weakening of the 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 SiH4(H-2)(2).