Theoretical Study Of Strange Electron Behavior And Superconductivity In Alkali Metal And Alkaline Earth Metal Compounds Under High Pressure

Pressure,as a fundamental thermodynamic variable,is often used to change the properties of materials,and it is a common means of producing exotic materials that cannot be obtained under environmental conditions.Alkali metal and alkaline earth metal compounds under high pressure have rich structure and strange electronic properties.Alkali metals are also known as the simplest metals because of their simple electronic structure.It has been reported that at one million atmospheres of pressure,alkali metals and alkaline earth metals will undergo an average of five phase transitions,or have five different spatial group structures.So alkali metals and alkaline earth metal compounds are abundant.And these compounds have excellent properties in energy storage,superconductivity,solid-state batteries and so on.Alkali metal compounds,especially alkali metal sulfides,have been extensively studied in energy storage and solid-state batteries due to their high ionic conductivity and large electronic band gap,and have shown good properties in superconductivity.The phase diagram,crystal structure,electronic properties and phonon characteristics of Na-S system under high pressure are systematically simulated.The results show that Na S,Na₂S and Na₄S are thermodynamically stable in the pressure atmosphere under study.Na₃S are thermodynamically stable at pressures greater than 30 GPa.In the above compounds,it can be concluded that the structure of Na S and Na₂S is the insulating property of non-metals through the band structure.Both Na₃S and Na₄S have metallic structures.Moreover,we calculate the superconductivity of Na₃S.Due to the weak electron-phonon coupling,the critical temperature of the superconducting transition（T_c）of Na₃S is estimated to be close to 0 K at 100 GPa.By comparing the structure,electronic characteristics and stoichiometric ratio of alkali metal ions of Li-S,Na-S,K-S and other alkali metal sulfides,we found that the chemical ratio and pressure of alkali metal atoms can promote the transformation of alkali metal sulfides from insulators to metals.Inspired by the electron rich properties of alkali metal sulfides,we studied the alkali metal compounds of O element in the same major group as S.In the Li-O system,we investigate the anomalous Wilson phase transition from a metallic to a non-metallic state in this simple electronic system under high pressure,which we believe is important for understanding the profound physical mechanisms related to the electronic structure of materials.And these studies have important reference value for understanding complex electronic systems.What is particularly interesting is that by calculating the electronic properties,we find that the Li-O system under high pressure shows different normal and abnormal Wilson transition behaviors according to the ratio of Li to O.And Li-O compounds also exhibit superconductivity.However,the pressure phase transitions and superconducting properties of these materials have not been fully investigated,especially for lithium rich oxides.In order to solve this problem,we use the first principles calculation to search and optimize the ab initio structural relaxation of Li_mO_nin the pressure range of 1 bar～200 GPa in the framework of density functional theory.Ten ratios with thermodynamic stability were obtained according to the formation enthalpies of their decomposition into other Li-O compounds or Li and O solids,and the detailed structural information of all the stable compounds was obtained.The structures we found in Li₂O₂have lower enthalpy of formation than those reported in the literature.By calculating the electronic properties,we found that the odd and even properties of m in Li-rich Li_mO compounds under pressure determine the metallization or anti-metallization properties of Li_mO compounds under high pressure,which is independent of the ratio of Li to O,that is,the magnitude of m.This finding has not been seen in other materials with Wilson transformations.Li₈O has superconductivity at the C2/c phase pressure of 100 GPa,the superconducting transition temperature（T_c）of 5.54 K,and exhibits antimetallization characteristics with the increase of pressure.This is different from the superconductivity of other Li-rich compounds and most hydrides,which is mainly affected by the density of electron states at the Fermi level.In contrast,the superconductivity of the C2/c phase Li₈O is mainly affected by the mean electron-phonon matrix element on the Fermi level with increasing pressure and the decrease of the mean phonon frequency with increasing pressure.And mainly from the Li atom’s contribution.In the study of the exotic electronic properties of alkali metals and alkaline earth metals,the discovery of Mg B₂superconductivity has attracted wide attention from researchers.The compounds reported by doping in Mg B₂have been observed with reduced superconducting transition temperature.However,it has been reported that through the addition of hydrogen atoms in two-dimensional monolayer Mg B₂,the superconducting transition temperature of the system can be improved by using the changes in electronic and vibrational characteristics caused by hydrogen atoms.This has aroused our interest in exploring the Mg B₂-H system.In the Mg B₂-H system,we found several stable ratio of Mg B₂doping H solid（Mg B₂）_mH（m=5,6,9,12）,and through calculation,the four structures are metallic.Therefore,the ratio of the selected solid solution is calculated.The results show that the density of states of the system is increased because of the addition of H,the critical transition temperature of Mg B_2mH（m=5,6,9）at 100 GPa is greater than the critical transition temperature of pure Mg B2 under the same pressure.This shows that the existence of H as gap doping in Mg B₂system can better affect the acoustic coupling properties of the system,which provides a new reference for improving the superconductivity and application of MgB₂ system in the future.