Study On New Structures And Superconductivity Of Hydrogen-rich Compounds Under High Pressure

Hydrogen is not only the lightest element in nature,but also the most abundant element in the Universe.According to the Bardeen-Cooper-Schrieffer theory,due to the small atomic mass of hydrogen,it has a high Debye temperature and strong electron-phonon coupling（EPC）,suggesting that“metal hydrogen”will be an ideal high-temperature superconductor.However,the pressure required to achieve pure hydrogen metallization is extremely high.However,hydrogen in hydrogen-rich materials can be metalized and exhibit superconductivity under lower pressure than pure hydrogen due to chemical or physical precompression effects.Therefore,hydrogen-rich materials have attracted wide attention for their unique physical properties of potential high-temperature superconductivity.Many novel physical and chemical properties usually can be found in materials under high pressure due to the interatomic distance reducing and the bonding patterns changing.The most common phenomenon is a structure will also undergo multiple structural phase transitions;besides,many unexpected materials exhibit high temperature superconductivity at high pressure.And metal hydrides are no exception.More hydrogen-rich superconducting compounds that are not available at atmospheric pressure have been predicted under high pressure,such as H₃S,La H₁₀,YH₉and Ca H₆,and confirmed by experiments.Therefore,predicting the structure and superconductivity of hydrogen-rich materials can help to find stable superconductors with high superconducting transition temperaturem（T_c）and provide some theoretical guidance for experiments.In this paper,the crystal structure,mechanical and dynamic stability,electronic properties,phonon properties and superconductivity of MH₂（M=Nb,Ta）and ternary Mg-Ti-H system under high pressure are predicted theoretically by using the method of crystal structure prediction and first-principles calculation.Some obtained theoretical research results are as follows:（1）The high-pressure structures and properties of MH₂（M=Nb,Ta）are explored through an ab initio evolutionary algorithm for crystal structure prediction and first-principles calculations.It is found that Nb H₂ undergoes a phase transition from a cubic Fm3?mm structure with regular Nb H₈ cubes to an orthorhombic Pnma structure with fascinating distorted Nb H₉ tetrakaidecahedrons at 48.8 GPa,while the phase transition pressure of Ta H₂ from a hexagonalm P6₃mc phase with slightly distorted Ta H₇ decahedron to an orthorhombic Pnma phase with attractive distorted Ta H₉ tetrakaidecahedrons is about 90.0 GPa.Besides,the calculated electronic band structure and density of states demonstrate that all of these structures are metallic.The Poisson’s ratio,electron localization function（ELF）and Bader charge analysis suggest that these phases possess dominant ionic bonding character with the effective charges transferring from the metal atom to H.From our electron-phonon calculations,the calculated superconducting critical temperature T_c of the Pnma-Nb H₂ is 6.903 K at 50 GPa.Finally,via the quasi-harmonic approximation method,the phase diagrams at pressure up to 300 GPa and temperature up to 1000 K of MH₂（M=Nb,Ta）are established,where the transition pressure of Fm3?m-Nb H₂→Pnma-Nb H₂ and P6₃mc-Ta H₂→Pnma-Ta H₂ was found to decrease with increasing temperature.（2）The stable structures of ternary hydride Mg-Ti-H at pressures of 50,100,150 and200 GPa has been performed by means of the evolutionary algorithm USPEX code and first-principles calculations.Here,we found several stable hydrides P6₃/mmc-Mg Ti H₂,R3?m-Mg Ti H₄,P2₁/m-Mg Ti H₅,Pmn2₁-Mg Ti H₆,Pmmn-Mg Ti H₆ and R3m-Mg₃Ti H₁₂ in the systematic investigation of Mg-Ti-H ternary hydrides under pressure.In particular,hydrogen in its structure mainly exists in the form of a single atomic hydrogen.Furthermore,the bonding nature of six stable phases are investigated with the help of the electron localization function and Bader charge analyses,which show that an ionic bond is formed between Mg-H and Ti-H with the charges transferring from metal atom to H.More importantly,the electron-phonon coupling calculation indicates that R3m-Mg₃Ti H₁₂is a promising superconductor with an estimated superconducting transition temperature of 71.185 K at 100 GPa,which is close to the superconducting transition temperature of90 K of ternary hydride（La,Y）H₄synthesized experimentally at 100 GPa.Further analysis shows that the phonon modes associated with H atoms contribute significantly to the electron-phonon coupling.Our present results provide guidance for future experimental studies and promote the synthesis of these ternary hydrides under high pressure.