Theoretical Study On The Structures And Properties Of Several New Hydrides Under High Pressure

Pressure, similar to temperature and component, is a thermodynamic parameter, which plays a decisive role in changing the structure and properties of condensed matter. Under pressure, the interatomic distances decrease, with the electron overlap intensified, which lead to phase transition. In addition, the pressure can reduce the chemical potential of the material, promote the formation of the reaction, and get some new materials which can not be synthesized under ambient condition. Furthermore, it can make non-superconductor into superconductor, and improve the superconducting transition temperature of the superconductor. Nowadays, high pressure has become an effective way to search for new functional materials.In 1935, Wigner and Huntington proposed that hydrogen might transform to a alkali-metal-like monatomic state when the pressure is higher than 25 GPa. The band gap will be narrowed by compressing, which makes the material from the insulator into a metal. Then in 1968, Ascroft pointed out that hydrogen might become a superconductor, owing to its small mass and high density of states. However, it hasn’t found the metallic hydrogen even the pressure up to 388 GPa in experiment. In 2004, Ascroft predicted that hydrogen-rich compounds might be metallic and become superconductors, which due to “chemical precompression.” Moreover, hydrogen-rich compounds contain a large number of hydrogen, so they can become potential hydrogen storage materials.In this work, we investigate the structures, the existence form of hydrogen, the metallization, superconductivity and superconducting transition mechanism of several new hydrides under high pressure by using genetic algorithm combining with the density functional theory（DFT）. Details as follow:1) Recently,it was found that the H3 S is a superconductor, and its Tc reaches 203 K（-70oC）, which breaks the previous record about copper-based superconductor（Tc = 164 K）. And the new finding has been stimulating people to investigate hydrogen-rich compounds. So we systematically study the polonium hydrides at high pressure. The following results as: Po H, Po H2, Po H4 and Po H6 stoichiometries become stable with increasing pressure. In addition, Po H2, Po H4 and Po H6 adopt stable structures with H2 units. We also find that Po H-P63/mmc, Po H4-C2/c, and Po H6-C2/m are superconductors with Tc of 0.14-0.65 K, 41.1-47.2 K and 2.25-4.68 K at different pressures.2) As we know, there are no indium hydrides at ambient condition. So we investigated the high-pressure structures, electron properties and superconductivity of In3 H, In2 H, and In Hn（n = 1-6）. Two stoichiometries（In H3 and In H5） are thermodynamically stable. Moreover, H2 or H3 units emerge in In H3 and In H5, which both show metallic characters. By the Bader analysis calculating, it is found that charges transfer from indium atoms to hydrides atoms. Furthermore, the electron-phonon calculations indicate that the Tc of R-3-In H3 and P21/m-In H5 are 34.1-40.5 and 22.4-27.1 K at 200, 150 GPa, respectively.3) It is reported that CH₄H₂ can be synthesized in experiment, and it keeps stable at least 30 GPa. Some datas about CH₄H₂ were given by experiment, however, many unanswered questions are still existent. So we study the structure and properties of CH₄H₂ under high pressure through ab initio evolutionary simulations. We found that in the CH₄H₂, CH4 and H2 remain the feature of the original molecular state. The phase transition is: P-1�'P212121�'P21/C. The three phases are insulators. Moreover, our results about the hardening behavior of H2-bond are in agreement with the experimental data. At the same time, it is found that the orientation disorder of H2 is disordered.4) The platinum group metals include platinum（Pt）, palladium（Pd）, osmium（Os）, iridium（Ir）, ruthenium（Ru） and rhodium（Rh）. Among of them, Rh, Pd, Ir and Pt hydrides have been studied either in experiment or theory at high pressures. However, both osmium and ruthenium hydrides are rarely known under high pressure. So we widely investigated the high pressure structures and properties of Os-H and Ru-H systems. For XHn（X= Os, Ru; n=1-8）, three stable stoichiometries（XH, XH3 and XH6） are predicted under pressure. And two species XH, XH3 show metallic feature, while XH6 is not. Interestingly, H2 units are found in XH6 stoichiometry. The superconductivity of the metallic Fm-3m-Os H, Fm-3m-Ru H, Pm-3m-Ru H3 and Pm-3n-Ru H3 was researched with Tc reaching 2.1, 0.41, 3.57 and 1.25 K at different pressures.5) Based on the reaches about the group IVB dihydrides Ti H2 and Zr H2 have been investigated either in theory or in experiment, so we examined the structures and superconductivity of Hf H2 system at zero temperature high pressure. We found that Hf H2 can occur phase transition at 180, 250 GPa, and its transition sequence is: I4/mmm�'Cmma�'P21/m. The three phases exhibit metallic feature with band overlap. In addition, charges transfer from the hafnium to hydrogen with ionic bonds forming in Hf H2. The Tc values for I4/mmm, Cmma, and P21/m reach 47-193 m K, 5.99-8.16 K and 10.62-12.8 K at different pressures.