| dcterms.abstract | Making solid metallic hydrogen proved elusive, even at pressures up to nearly 400 GPa. An alternative approach is to use chemical alloying as a means to exert additional pressure on hydrogen atoms. Following the recent breakthrough in predicting and synthesizing hydrogen sulfide with record-high Tc = 203 K [Drozdov et al., Nature (2015)] and with the motivation of discovering high-temperature superconductors, evolutionary algorithm USPEX is employed to search for all stable compounds in the group-IV hydrides. This method has been successfully used to predict new materials that were confirmed experimentally, e.g., Zhang et al., Science (2013), Mannix et al., Science (2015) and Oganov et al., Nature (2009). 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 were systematically investigated. I4¯ m2-SnH8, C2/m-SnH 12 and C2/m-SnH14exhibit 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 I4¯ m2-SnH8 and novel linear H4 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 H 3. In the Ge-H system, in addition to the earlier structure of germane with space group Ama2 (which we showed to become a high- Tc superconductor [Gao et al., Phys. Rev. Lett. (2008)]), we propose a new C2/m structure, which is energetically more favorable at pressures above 278 GPa (with inclusion of zero-point energy). Our calculations indicate metallicity of the new C2/m phase of germane with Tc = 67 K at 280 GPa. Germane is found to exhibit thermodynamic instability to decomposition into hydrogen and the new compound Ge3H 11 at pressures above 300 GPa. Ge3H11 with space group I4¯m is found to become stable at above 285 GPa and is a superconductor with Tc = 43 K. We find that the pressure-induced phasestability of germanium hydrides is distinct from its analogous isoelectronic systems, e.g., Si-hydrides and Sn-hydrides. Prior to H3S, the highest experimentally observed Tc in conventional superconductors, which obey the BCS theory was in MgB2, which opened avenues for searching for higher Tc superconductors. We performed ab initio evolutionary searches for all the stable compounds in Mg-B binary system in the pressure range of 0-200 GPa. We found previously unknown, yet thermodynamically stable, compositions MgB3 and Mg3B10. Experimentally known MgB2 is stable in the entire pressure range 0-200 GPa, while MgB7 and MgB12 are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB4, which becomes unstable against decomposition into MgB2 and MgB 7 at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB2-type structure. However, with the exception of MgB2, all other magnesium borides have low electron-phonon coupling constants λ of 0.32 to 0.39 and are predicted to have Tc below 3 K. Very unusual chemistry and high-Tc superconductivity make this new family of compounds very interesting for a broad readership including chemists, physicists and materials scientists. | |
