A First-Principles Study Of Silane (SiH₄) And Disilane (Si₂H₆) Under High Pressures

Ashcroft suggested that metallic hydrogen could be a high-temperate superconductor in 1968 after Wigner and Huntington proposed that solid hydrogen would become a quantum metal under sufficient compression in 1935. Later, a high Tc of metallic hydrogen of the order of 102 K was further proposed by Papaconstantopoulos et al., Min et al. , and Barbee et al.. Nevertheless, although metallization of hydrogen was predicted for decades, it still is an elusive goal of physics to this day. Because hydrogen has stubbornly remained insulating in diamond-anvil-cell experiments even under 342 GPa.After the discovery of the relatively high superconducting Tc in lithium, calcium and even the extensional light-element combination of MgB2 under pressure, a great deal of effort has been taken to seek an alternative route to hydrogen metallization. So the dense, metallic hydrogen-rich compounds were pointed out the likely candidates for high temperature superconductors.Furthermore, Ashcroft et al. recently suggested that the group IVA hydrides, methane (CH4), silane (SiH4), germane (GeH4) and stannane (SnH4), might undergo a transition to metallic and superconducting state at moderate pressures than solid hydrogen because the hydrogen in these materials is"chemically precompressed"due to the presence of the group IVA atoms in the crystals. Methane the first candidate explored is found no clues of metallization up to 400 GPa by both theoretical and experimental work recently. Although there has also been interesting theoretical works on germane and stannane, the most experimental and theoretical investigations focused on silane have been undertaken to confirm the prediction. Feng et al. obtained the Pmna phase of silane by calculating the enthalpies of several possible structures under high pressure with a critical superconducting temperature (Tc) of 166 K at 202 GPa. Two phases of I41/a and C2/c SiH4 from a random strategy were obtained by Pickard et al.. By simulated annealing, Yao et al. obtained another C2/c phase of silane with the Tc of 45 ~ 55 K and the stable range of 90 ~ 125 GPa. Chen et al. obtained the superconducting structure of Cmca over a wide pressure range above 60 GPa with the Tc of 20 ~ 75 K. Between the stable range of 220 ~ 250 GPa, Miguel Martinez-Canales et al. obtained the superconducting structure of Pbcn with a Tc of 16K by an ab initio evolutionary algorithm. The recent noteworthy experiments have found that silane can form a metastable metallic phase above 60 GPa and exhibits peculiar superconductive features But the metallic P63 phase of silane based on x-ray diffraction proposed by Eremets et al. is found to be dynamically unstable with imaginary phonon branches by Chen et al. and Miguel Martinez-Canales et al. Subsequently, Olga Degtyareva et al. point out that the x-ray diffraction is not due to the"metallic hcp phase of silane"but PtH, which forms upon the decomposition of silane and reaction of released hydrogen with gasket or electrode metal (e.g. platinum) in the sample chamber. The pressure-induced metallization of silane in the moderate range of high pressure is still under debate, and the relevant peculiar superconductivity observed in the silane experiment remains a puzzle.In this work, we investigate the superconductive behaviors of the possible materials exist in the silane experimental sample by the first-principles calculations. Our results show that pure silane is stable and does not decompose under high pressure. But silane reacts with Pt and participates in the formations of platinum hydrides, Si and hydrogen, and these substances can be around Pt gasket and electrode under high pressure. The increasing of the superconducting transition temperature Tc with increase pressure below 100 GPa can be explained by the superconductivity of possible amorphous Si and platinum hydrides (P63/mmc and P-3m1 phases). For understanding the peculiar pressure dependence of Tc that decrease first and then increase of Tc with pressure above 100 GPa, a new phase of silane with peculiar superconductivity must be involved. We suggest a superconducting P21/m phase of silane. The peculiar superconductivity can be understood by the PtH platinum hydride and the new silane which brings the increase of Tc with pressure. Furthermore, we investigate the relevant mechanism of the peculiar superconductivity. Our calculations demonstrate that the monotonic drop of﹤I2﹥(the average of squared electron-phonon coupling interaction) with pressure in PtH causes the decrease of Tc above 100 GPa. The metallic phase of P21/m silane shows a peculiar superconductive behavior that its electron-phonon coupling (EPC) parameter and Tc decrease initially and increase later with the increasing pressure in its dynamically stable range. Different EPC mechanisms are uncovered during the increase and decrease of the EPC parameter. Perturbative linear response calculations demonstrate that the decrease of EPC parameter mainly comes from the low-frequency acoustic vibrational mode becoming hardening before 125 GPa and the increase mainly from the Fermi surface topology transitions after 125 GPa. The low FS filling constant and inefficient FS outlines depicted by the Fermi surface nesting function mostly account for the unexpected low Tc.It was recently predicted that group IV hydrides would present a high superconducting critical temperature, while becoming metallic at lower pressures due to chemical precompression. Both the theoretical and experimental studies of silane, and the theoretical studies of germane and stannane, have investigated possible metallization and superconducting phase transitions at high pressures, as mentioned above. Indeed, the theoretical studies on germane and stannane have predicted very high Tc of 64 K at 220 GPa and 80 K at 120 GPa, respectively. These results obtained are sufficiently encouraging us to prompt studies on a wider range of hydrides to confirm the prediction. Disilane containing a large fraction (3/4) of H atoms is also an important hydrogen-rich compound and lead to interesting properties under high pressure. Furthermore, it is more readily available for experimental studies because of the higher boiling and melting points than silane, germane and stannane. However, studies on disilane are very scarce.Here, we have used a random structure-searching method to explore the crystal structures of disilane in a wide pressure range from 50 to 400 GPa. Three metallic structures, i.e. P-1, Pm-3m and C2/c, are found here and energetically much superior to those of XY3-type candidates under high pressure. We have revealed a wide decomposition pressure range from 50 to 135 GPa, above which these three structures are thermodynamically, mechanically, and dynamically stable. Perturbative linear-response calculations for the three structures are performed at selected pressures. Tc of the P-1 phase at 175 and 200 GPa are 65 and 80 K. For C2/c phase at 300 GPa, the estimated Tc is 34 K. Remarkably, the estimated Tc of Pm-3m phase at 275 GPa reaches a very high value of 139 K, a Tc beyond the order of 102 K.