Hydrogen segregation and its roles in structural stability and metallization: Silane under pressure

Abstract

We present results from first-principles calculations on silane (SiH 4) under pressure. We find that a three dimensional P-3 structure becomes the most stable phase above 241 GPa. A prominent structural feature, which separates the P-3 structure from previously observed/predicted SiH 4 structures, is that a fraction of hydrogen leaves the Si-H bonding environment and forms segregated H 2 units. The H 2 units are sparsely populated in the system and intercalated with a polymeric Si-H framework. Calculations of enthalpy of formation suggest that the P-3 structure is against the decomposition into Si-H binaries and/or the elemental crystals. Structural stability of the P-3 structure is attributed to the electron-deficient multicenter Si-H-Si interactions when neighboring silicon atoms are linked together through a common hydrogen atom. Within the multicenter bonds, electrons are delocalized and this leads to a metallic state, possibly also a superconducting state, for SiH 4. An interesting outcome of the present study is that the enthalpy sum of SiH 4 (P-3 structure) and Si (fcc structure) appears to be lower than the enthalpy of disilane (Si 2 H 6) between 200 and 300 GPa (for all previously predicted crystalline forms of Si 2 H 6), which calls for a revisit of the stability of Si 2 H 6 under high pressure.