Abstract
With the motivation of discovering high-temperature superconductors, evolutionary algorithm USPEX is employed to search for all stable compounds in the Sn-H system. In addition to the traditional SnH4, new hydrides SnH8, SnH12 and SnH14 are found to be thermodynamically stable at high pressure. Dynamical stability and superconductivity of tin hydrides are systematically investigated. Im2-SnH8, C2/m-SnH12 and C2/m-SnH14 exhibit higher superconducting transition temperatures of 81, 93 and 97 K compared to the traditional compound SnH4 with Tc of 52 K at 200 GPa. An interesting bent H3–group in Im2-SnH8 and novel linear H in C2/m-SnH12 are observed. All the new tin hydrides remain metallic over their predicted range of stability. The intermediate-frequency wagging and bending vibrations have more contribution to electron-phonon coupling parameter than high-frequency stretching vibrations of H2 and H3.
Highlights
With the motivation of discovering high-temperature superconductors, evolutionary algorithm USPEX is employed to search for all stable compounds in the Sn-H system
Reaching the metallic state in pure solid hydrogen proved elusive, it is in the main focus of many groups and recently, the progress of bringing pure hydrogen to nearly 400 GPa has been reported[3,4,5]
Following the pioneering work of Ashcroft[6], nearly room-temperature superconductivity was predicted in metallic molecular hydrogen[7,8]
Summary
Evolutionary variable-composition searches for stable compounds and their structures with up to 20 atoms in the primitive unit cell were performed at 150, 200, 250 and 300 GPa. In the I4m2-SnH8 structure predicted to be stable at pressures above 220 GPa, Sn atoms are packed between H2 and H3 molecular groups, in which the bent H3 units are perpendicular to one another and sepated by 1.35 Å. We checked the effects of zero-point energy using phonon calculations[25] at 250 GPa. The inclusion of zero-point noticeably lowered the formation enthalpy of SnH8 with respect to SnH4 and H2 (Fig. 1(a)), implying that this compound can be formed at lower pressure. SnH14, reported for the first time in this work, are thermodynamically stable compounds that coexist stably with solid Sn, H2 and SnH4 in a wide pressure range starting from 220 to at least 300 GPa. EPC calculations indicate that high-pressure SnH8, SnH12 and SnH14 are phonon-mediated superconductors with Tc values of 81, 93 and 97 K at pressures of 220, 250, and 300 GPa, respectively.
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