Pressure induced metallization of SiH4(H2)2 via first-principles calculations

Structure of SiH4-H2, which has attracted much theoretical and experimental effort, but there remains many unsolved problems, for example, behavior of H2 under compression, as well as the metallization mechanism induced by pressure. In this work, two competitive structures of SiH4(H2)2 were studied to find the stable structure in the pressure range of 6.8-35 GPa. Our results indicate that tI18 structure is the possible structure, which can also be viewed as a distorted face-centered-cubic lattice. The absence of imaginary frequency modes for phonon dispersion curves indicates that tI18 structure is dynamically stable in the pressure range of 6.8-35 GPa. The optimized structure is also in good agreement with experimental structure, supported by X-ray diffraction data. Specially, the calculated low-frequency phonon modes and electronic band structure show that this crystal becomes metallic at 190 GPa and 0 K, and the potential superconductivity in this compound can be attributed to a conventional electron-phonon mechanism that couples the low-frequency phonon mode to the sp-like conduction electrons. The electron density difference is also analyzed to have a further comprehension on pressure-induced metallization mechanism of this compound. Contrasting with the general view in the previous studies that the metallization may arise from the interaction of SiH4 and H2, it is found the interaction between H atoms in SiH4 and H2 molecules is the main factor, that induces the metallization or even superconductivity of SiH4(H2)2 under high pressure.