First-principles Studies On High Pressure Phase Diagram And Superconductivity Of Li-Mo-H System

Pressure（pressure）is one of the fundamental thermodynamic variables and has an important influence on the structure and properties of substances.In the process of the pressure changes,the volume of substances changes at the macro level,and on microscopic level,nucleus and electrons in the material are rearranged in response to change in pressure.During this process,substances may undergo phase transitions,transform into new structures,and even produce chemical reactions to form new compounds.This indicates that regulating pressure is an important means for finding,designing,and synthesizing new substances.At the same time,pressure is an important means to achieve metallization of substances.As early as 1935,Wigner and Huntington predicted that insulated molecular hydrogen would be converted to metal atom hydrogen under high pressure.According to BCS theory,the transition temperature of superconductors is positively correlated with the Debye temperature,which is inversely proportional to the atomic mass.Hydrogen is the lightest element,so metallic hydrogen is considered as a potential superconductor at high temperature or even room temperature.However,metallic hydrogen requires extremely high pressures to achieve,which makes experiments difficult.In order to reduce the pressure required for hydrogen metallization,Ashcroft proposed a"chemical precompression"scheme in 2004,which involves introducing non hydrogen elements into pure hydrogen to produce a metal phase with an electronic structure dominated by hydrogen at the Fermi level,and utilizing interatomic interactions to achieve the goal of reducing stable pressure,so as to achieve high temperature superconductivity at low pressure.Currently,a large number of high-temperature superconducting hydrides have been successfully predicted and synthesized,demonstrating the great potential of hydrides in high-temperature superconductivity.Among them,the most representative are covalent hydrides H₃S and clathrate hydrides La H₁₀,with the highest superconducting transition temperatures measured experimentally reaching 203 K and 260 K,respectively.Introducing new elements into binary hydrides to form ternary hydrides is one of the important means to improve existing hydrides.It is generally believed that the electrons of the H₂molecular unit are far away from the Fermi level,which is detrimental to superconductivity.It is generally believed that the electrons of the H2molecular unit are far from the Fermi level,which is not conducive to superconductivity.It was found that Fd3 m-Li₂Mg H₁₆formed by incorporating low electronegative alkali metal Li into the Mg H₁₆structure with a large number of H₂molecular units,and its structure had a superconducting transition temperature of up to 473 K at a pressure of 250 GPa.In Li₂Mg H₁₆,metal atoms transfer electrons to hydrogen atoms,and the electrons transferred to hydrogen occupy the anti-bonding orbitals of H₂molecular units,causing H₂molecular units to dissociate and form cage type hydrogen,thereby increasing the superconducting transition temperature.It is reported that Cmmm-Mo H₁₁has a superconducting transition temperature of 165-182K at 250 GPa,and there are atomic H,H₂molecular units,and H₃molecular units within the structure.In this work,the alkali metal lithium is introduced into the Mo-H system,enabling the H₂and H₃molecular units to obtain sufficient electrons and then decompose into atom H.It is expected that the superconducting transition temperature of the system will further increase.In this paper,the first principles calculation method and crystal structure prediction technology are used to theoretically predict the crystal structure,phase diagram,electronic structure,and superconductivity of the Li-Mo-H system under high pressure.Combining enthalpy difference diagrams with crystal dynamics calculations,we have found many stable chemical ratios,such as Li Mo H₃,Li Mo H₅,Li Mo H₆,Li₄Mo H₉,and Li₃Mo₂H₁₄.At the same time,we found the metastable structure Li₄Mo H₁₅.Among all dynamically stable structures,C2/m-Li₄Mo H₉and Immm-Li₄Mo H₁₅exhibit excellent superconductivity,with superconducting transition temperatures of 75-84 K and 111-124 K at 200 GPa,respectively.To analyze its superconducting mechanism,we conducted research and analysis on the electronic properties,interatomic bonding,and superconductivity of Li₄Mo H₉and Li₄Mo H₁₅.The electron localization function diagram shows that all H in the Li₄Mo H₉structure is completely dissociated and exists in the form of ion H;the H part in the Li₄Mo H₁₅structure dissociates and exists in the form of a mixture of H ions,H₂quasi-molecule unit,and H₂molecules.In the system studied in this work,the H₂quasi-molecule unit is a key factor affecting high-temperature superconductivity.If a metal hydride system wants high-temperature superconductivity,it cannot lack transfer electrons to cause hydrogen to exist in molecular form,nor can it introduce too many transfer electrons to cause hydrogen molecules to completely dissociate into hydrogen ions.Charge transfer needs to be precisely controlled so that a long H-H bond is maintained between H,which produces strong electroacoustic coupling,resulting in a high superconducting transition temperature in the system.This work not only studies the phase diagram of the Li-Mo-H system in detail,but also enriches the database of high-temperature superconducting hydrides.At the same time,it also provides an important reference for the study of the superconductivity of transition metal hydrides and the dissociation of H₂molecular units.