Theoretical Studies On Several Hydrogen-based High-temperature Superconductors Under High Pressure

The unique properties of superconducting materials make them have broad application prospects,and room temperature superconductivity has been a dream pursued by mankind for more than a century.Based on the BCS theory,the superconductivity transition temperature（T_c）of substances is positively correlated with the Debye temperature,and the Debye temperature is negatively correlated with the atomic mass.Therefore,selecting lighter elements can effectively improve the superconductivity transition temperature.As the lightest and widely existing element,hydrogen will undoubtedly be a powerful candidate for room temperature superconductors after metallized.However,pure hydrogen metallization requires a pressure of at least 500 GPa.The"chemical precompression"can significantly reduce the pressure required for hydrogen metallization by introducing non-hydrogen elements.Therefore,hydride has become an important exploration direction to realize high-temperature superconductors.Hydrides have simple crystal structures and are conventional superconductors based on BCS theory.Therefore,under the guidance of first-principles calculation,the research of hydrogen-based high-temperature superconductors has made a series of breakthroughs rapidly.For example,the theoretical prediction of H₃S and La H₁₀ has been confirmed by experiments to have a superconductivity transition temperature of more than 200 K at 150 GPa,which has set a new record of high-temperature superconductivity and stimulated people’s enthusiasm to search for high-temperature superconductors in hydrides.Based on the density functional theory,combined with the first-principles calculation method and the crystal structure prediction method,the author has studied several hydrogen-based high-temperature superconductors:through the doping of heavy rare earth element Yb/Lu,several clathrate superhydride room temperature superconductors that can be stable under moderate pressure have been designed,and the role of charge transfer and bond length distribution in enhancing electron-phonon coupling has been demonstrated;The superhydride Mo H₁₁ with transition metal Mo was designed,and the effects of acoustic and optical modes on electroacoustic coupling were explored;The high pressure phase diagram of CS₂H₁₀ is predicted,and the effect of CH₄ insertion on the superconductivity and stable pressure of H₃S lattice is studied;The novel high pressure phase of YH₉ is predicted,which well explains the inconsistency between the theory and the experiment of the trend of superconducting transition temperature changing with pressure.The specific research results are as follows:（1）Room-temperature superconducting Yb/Lu substituted clathrate hydrides under moderate pressure.Room temperature superconductivity has been a dream pursued by mankind for a century.In recent years,the synthesis of H₃S and La H₁₀ under high pressure has gradually made this dream almost come true.However,the high pressure（above 150 GPa）required to stabilize these hydrogen-based superconductors limits its application.At present,the important challenge we are facing is how to realize room temperature superconductivity at low pressure or even ambient pressure.Recent studies have shown that rare earth elements with4f orbitals can produce a stronger"chemical precompression"effect,and when the f electrons further fill the 4f orbitals to completely occupy,the heavy rare earth metal Yb/Lu can also show excellent superconductivity in sodalite-like clathrate hexahydrides Yb H₆ and Lu H₆.Inspired by this,author added the heavy rare earth element Yb/Lu to the sodalite-like hexahydride,and designed a series of high-temperature superconductors that can be dynamically stable under moderate pressure.In particular,Y₃Lu H₂₄,YLu H₁₂ and YLu₃H₂₄ can reach 283 K at 120 GPa,275 K at 140GPa and 288 K at 110 GPa respectively.Their critical temperature is close to or has reached room temperature,and the minimum stable pressure is significantly lower than the pressure of room temperature superconductors reported previously.This study provides an effective method for the rational design of low pressure stable hydrogen-based superconductors with high T_c,and will stimulate further experimental exploration.（2）High-temperature superconductivity in transition metallic hydrides MH₁₁（M=Mo,W,Nb,and Ta）under high pressure.In the study of layered disulfide superconductors,transition metals Nb,Ta,Mo and W are the most popular doping elements.Their layered disulfide superconductors can show excellent properties.These metals and hydrogen forming hydrogen-rich compounds are also likely to be potential high-temperature superconductors.The author found three novel stable compounds in Mo-H system:Mo H₅,Mo H₆ and Mo H₁₁.The hydrogen-rich phase Cmmm-Mo H₁₁ has a layered structure and contains various forms of hydrogen:H atom,H₂^-and H₃^-molecular units.It is a high temperature superconductor,and its T_c is estimated to be between 165-182 K at 250GPa.The same structure was also found in Nb H₁₁,Ta H₁₁ and WH₁₁,with T_c between 117 to 168 K.By combining two coupling constantsλ_opt andλ_acand two characteristic frequencies（optical and acoustic）with the first-principle calculation.It is found that the high T_c is mainly caused by the existence of high-frequency optical modes,and the acoustic mode also plays an obvious role.These results provide a reference for the future study of transition metal hydride superconductivity.（3）Theoretical study on crystal structure and superconductivity of CS₂H₁₀ under high pressure.It has been reported that high temperature superconductivity with a critical temperature of 288 K has been observed in the compounds of hydrogen,carbon and sulfur under high pressure,but the chemical composition and crystal structure of this room temperature superconductive phase have not been determined.Subsequent studies have extensively explored the C-S-H system.The results show that the structure of carbon-rich and hydrogen-rich is not a low-enthalpy phase,it is easy to decompose into H₂,H₃S and CH_x.In the whole C-S-H ternary phase diagram,only CS₂H₁₀ has lower enthalpy.Therefore,the author studied the high pressure phase of CS₂H₁₀ and found four new phases:Cmc2₁,P3m1,P-3m1 and Pm.Among them,P3m1 can be dynamically stable at a minimum of 50 GPa,while Cmc2₁ has a high T_c of 155 K at 150 GPa.Both Cmc2₁ and P3m1 are CH₄-H₃S host-guest hydrides,which correspond to the structure formed by CH₄ molecule inserting into Im-3m-H₃S and R3m-H₃S sublattice,respectively.Their T_c is dominated by H₃S lattice.The insertion of CH₄ greatly reduces the pressure required for the stability of the H₃S lattice,but it has a negative impact on superconductivity that cannot be ignored.By studying the effect of CH₄ insertion on H₃S lattice,the hydride with T_c close to H₃S can be designed,and its pressure required for stability can be greatly reduced.By comparing with the experimental P-V diagram,the author believes that the phase obtained in the experiment is neither the CH₄ inserted H₃S phase nor the C-doped H₃S phase.Only the highly hydrogen-rich C-S-H compound can be consistent with the C-S-H compound in the experiment on the P-V diagram.（4）Theoretical study on structural distortion and superconductivity of YH₉ under high pressure.Yttrium hydrogen compounds have attracted wide attention due to their rich proportions,diverse crystal structures and excellent superconductivity under high pressure.In particular,the T_c of YH₉ is up to 243 K,second only to La H₁₀ among all superconductors.The T_c measured in the experiment below 200 GPa shows positive pressure dependence,which is contrary to the theoretical estimation.In order to explore the source of T_c at low pressure and explain the difference between experiment and theory,the author systematically explored the crystal structure of YH₉ at different pressures and found a twisted cage structure with Pnma space group.The phase is the most stable in thermodynamics under the pressure of less than 220 GPa,and its X-ray diffraction pattern is consistent with the experimental data.Most importantly,T_c of Pnma phase increases with pressure,which is consistent with the experimental results.Further calculations show that the structural distortion strongly affects the lattice vibration and electron-phonon coupling,resulting in a positive T_cpressure dependence of the Pnma phase.