| Seeking for high temperature superconducting materials has been one of the hot spots and frontier research topics in condensed matter physics. According to the BCS theory, the superconducting transition temperature varies inversely with the atomic mass. So scientists speculated that hydrogen, which possess the attribute of the smallest relative atomic mass, may have a higher transition temperature after metallization. Therefore, the exploration of metallization and superconductivity of hydrogen has become a significant research for seeking the traditional superconductors. However, hydrogen is impossible to be a superconductor at ambient pressure. While compression may be an effective exploring method, it can change the electronic structures, then the phase transition from an insulator to a metal takes place. Thus, the studies of structures and properties of hydrogen under high pressure become favorable breach to find the room temperature superconductor. So far, hydrogen has been predicted with high superconducting transition temperature of 100-700 K under high pressure in theory. Unfortunately, it still has not been observed up to 380 GPa in experiment.In 2004, a new proposed bypass by Ashcroft is “chemical precompression” method by doping with particular elements, which can reduce the pressure of metallization. These ideas will challenge researchers to find a route for the metallization of solid hydrogen. Recently, several first-principles calculations and an experiment indicated high superconducting transition temperature about 200 K in the sulfur hydrides system. These fruits persuasively demonstrated the effectiveness of the “chemical precompression” and reignited great interests in exploring hydrides under high pressure.The group IV hydrides are also good candidates that experienced extensive literature from both experimental and theoretical sides. So far, many of the group IV hydrides(such as Si H4, Si2H6, Si H8, Ge H4, Ge H8, Sn H4, Pb H4 et. al.) are predicted to be metallization with lower pressure and high superconducting transitation temperature, many of which break through 100 K. Hence the group IV hydrides are crucial breach for searching potential high temperature superconductors. Although it has experienced a lot of researches, there are still have many problems, such as whether exists other structures expect the most concerned tetravalent hydrides or what are their superconducting transition mechanism. In this context, we firstly probed the hydrides of silicon, germanium, tin and plumbum.With regard to Si H4, we continue to investigate the high pressure structures thoroughly. We propose two new stable metallic structures of silane with P21/c and C2/m symmetries, also with the previously reported C2/c structure. The three structures are discovered to be superconductor. The trend of Tc with pressures for these three structures was explored for further investigation. They are all gradually decreasing with the pressure up. The tendency of the two parameters and that made λ become lower and lower, which lead to the decrease of Tc with increasing pressure. We also realized that Tc magnitude of C2/m was beyond 102 K which is higher than other two phases that promoted us to explore the underlying superconducting mechanism. By comparing the effects of,, and, we can conclude that in C2/m is much higher than the ones in other two structures. So we can get that play an important role. In this part, we provide new structures of silane and analyze the possible mechanism of high Tc, which has a certain significance for the improvement of the structure and analysismechanism of the traditional superconductors.The germane is extensively explored under high pressure by the first-principles calculations. Two new structures with space groups of Ama2 and C2/c are screened out by comparing the enthalpies and the compositional stability with ZP correction in view. They are found to be stable in the pressure ranges of 190-310 GPa and above 310 GPa with H2 and H3 units, respectivety. The characteristics of electronic structure are further explored. It is noteworthy that the localized electrons are around H2 and H3 units, which implies H-H is combined with covalent bond. In addition, the conductivity comes from the contributions of the electrons around the hydrogen atoms, where the value of ELF is around 0.5. Furthermore, the electron-phonon coupling calculations predict that both phases are superconductors with high Tc of 47-57 K for the Ama2 at 250 GPa and 70-84 K for C2/c at 500 GPa. Interestingly, the Tc of Ama2 Sn H4 is 15-22 K which was significantly lower than that of Ama2 Ge H4, which may be relevant to the different atomic mass of Ge and Sn. The speculation remains to be confirmed after the exploration. In this part, we updated the phase diagram of Ge H4 and provided the potential high Tc structures, which can provide guidance to the experiment.In Si-H and Ge-H system, there are several different components together under high pressure. As one of the elements closely correlated to Si and Ge, only Sn H4 has been investigated up to now. Therefore, it is necessary to carry out more comprehensive and accurate variable components prediction of Sn-H systerm. A high-pressure phase diagram of the hydrogen-rich Sn-H system was established by structure prediction simulations and first-principles calculation. According to our investigation, no Sn-H crystals appeared below 131 GPa. Afterwards, the previously predicted Sn H4 with Ama2 and P63/mmc symmetries appeared at 131-150 GPa and above 150 GPa, respectively. Moreover, a hitherto unknown stoichiometry Sn H8 with I-4m2 symmetry is also uncovered stable at pressures of 238 GPa to 350 GPa. All H atoms of Sn H8 are in the form of H2 or H3 units with electrons localized around them, suggesting covalent bond character. Furthermore, lattice dynamics and electron-phonon coupling calculations indicated that the I-4m2 phase is a superconductor with high Tc of 63-72 K at 250 GPa, which derives from strong EPC λ. In addition, we also study the existence of Sn H4 under high pressure. Excepting for the predicted Ama2 and P63/mmc crystal structures, a new phase with C2/m symmetry was obtained at pressures above 400 GPa. Further calculations proved that the superconducting transition temperature could reach 64 K at 500 GPa, which was attributed to high values of the electron-phonon coupling parameter mainly contributed by H vibration modes. In this study, we proposed the phase sequence of Sn-H system. Structures of Sn H4 and new composition Sn H8 were predicted, which make sense to the further studies of Sn-H system.As the heaviest element of the group IV, Pb combination with the lightest H experienced relatively little works. In contrast to the ample investigation of Ge-H and Sn-H systems with different components, it is mainly concentrated on the study of Pb H4. That promote us to give a detailed exploration to update the Pb-H phase diagram with different Pb/H ratios. The crystal structures, phase diagram and properties of plumbum hydrides are investigated by using first-principles calculations. Several stoichiometries Pb H, Pb H2, Pb H3, Pb H5, Pb H6 and Pb H8 instead of the common tetravalent hydride are predicted with the dominant enthalpy values. All the predicted structures possess H2 units. After further dynamic stability determination, two Pb H8 structures with C2/m and Fddd symmetries are identified as stable ones. Both structures are metallic with electrons existed at Fermi level, showing complex 3D Fermi surface features. ELF analysis show strong electron localization around the H2 units, which are characterized by strong covalent bonds. Further electron-phonon coupling calculations manifest that both structures are superconductor with Tc of 59-69 K for C2/m at 180 GPa and 88-98 K for Fddd at 200 GPa, which mainly derived from the contribution of H atoms.The rare-earth hydrides have many special properties under high pressure, such as the superconductivity in relatively low pressures. In the investigation of yttrium hydride, fcc-YH3 was predicted to be a superconductor with Tc of 40 K at 17.7GPa, which is the lowest reported pressure for high superconductive hydrides up to now. Therefore, it is also an important direction to seek the potential high Tc materials in the lanthanide hydrides.As to REHx(RE=Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Lu) system, especially the REH3 compounds, the experimental and theoretical researches mainly concentrated on exploring the crystal structure, optical properties, pressure-induced phase transition and superconductivity and so on. Among the rare-earth hydrides, YH3 and the variable component compounds of Y-H system have been far-ranging studied, while the other rare-earth hydrides research are relatively few. Er-H and Ho-H systems may have similar properties with Y-H, however, the exploration mainly concentrated in the Er H3 and Ho H3. It was reported that they transformed from hcp(hexagonal-closed pack) to fcc(face-centered cubic) phase, but the hexagonal phase structure remains controversial. So we believe that the research of Er-H and Ho-H system is not impeccable, based on the above-mentioned problems, we gave an exploration on the Er-H and Ho-H systerm under high pressure.We searched for the thermodynamically stable structures of various stoichiometries of Er Hn and Ho Hn(n=1-6) using ELoc R code. In terms of the thermodynamic stability analysis, we proposed several stoichiometries REH, REH2, REH3, REH4 and REH6(RE= Er, Ho). Moreover, the phase sequence of Er-H and Ho-H is almost similar, indicating that the two systems may possess identical symmetry and analogous properties. The phonon dispersion curves along several high symmetry directions have been calculated and demonstrated that Er H(Fm-3m),Er H2(Fm-3m),Er H3(Fm-3m) and Er H4(I4/mmm) are dynamic stability. Furthermore, the electronic properties analysis of the above-mentioned Er-H structures show that all of them are metallic with large total DOS appeared at Fermi level, which is a favorable condition to promote Tc. |
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