High-pressure Phase Transition And Superconductivity Of Ternary Hydrides

The crystal structure of a substance determines its physical and chemical properties,so the crystal structure of a substance becomes an important basis for people to design materials.Materials with desired properties such as superconductivity can be obtained through crystal structure design.In recent years,superconductors have been continuously studied by scientists because of their unique electromagnetic properties.With the rapid development of computers,the crystal structure prediction of materials has also developed rapidly.People begin to use the method of crystal structure search to predict and seek superconducting materials at high temperature and even room temperature.According to the theory of BCS（Bardeen-Cooper-Schrieffer）,the superconducting transition temperature of a substance is proportional to its Debye temperature,which in turn is inversely proportional to the relative atomic mass of the substance,so elements with smaller masses are more likely to have higher superconducting transition temperature.Therefore,hydrogen has been widely studied as the lightest element in relative atomic mass.However,hydrogen under normal pressure is an insulator,resulting in its lack of superconductivity.As a basic thermodynamic variable,pressure can have two basic effects on materials:on the one hand,it can change the atomic spacing of the material,so that the atoms can be rearranged to produce new physical and chemical properties;on the other hand,it can change the crystal of the material.Usually in a high enough pressure environment,the non-metallicity of hydrogen at atmospheric pressure can be changed,and the hydrogen can acquire a new phase.The combination of hydrogen and other elements under high pressure can produce some superconductive compounds,such as H3S,La H10,etc.Although these hydrides are superconductive at high pressure,their synthesis is difficult due to the experimental difficulty in controlling the experimental conditions.Scientists use the crystal structure search prediction method combined with first-principles calculations to theoretically predict the superconductivity of binary and even ternary hydrides.The experiments are guided by the results of theoretical predictions and the loss of some experimental equipment is reduced.This paper mainly uses the self-compiled CRYSTREE（Crystal Structure Prediction based on Machine Learning）to search and predict the structure of Li₂SiH₆under high pressure,and combines first-principles calculations to study its electronic energy bands,electronic density of states and electronic phonons under different pressures.coupling,etc.The main work of this paper is as follows:（1）The crystal structure search was performed using the self-compiled CRYSTREE software.CRYSTREE crystal structure search is a software based on machine learning algorithms.When performing a crystal structure search,first generate an initial structure with random symmetry,then optimize the initial structure through density functional theory,then collect and analyze the enthalpy value of the structure based on machine learning,further optimize the structure,and finally converge according to whether the set enthalpy value is reached.And loop to calculate the step size criterion to end the structure search and get the final result.When using CRYSTREE and CALYPSO（Crystal Structure Analysis by Particle Swarm Optimization）at the same time to perform structure search on the same B-N system at 300 GPa,it is found that CRYSTREE has faster time efficiency and lower time efficiency than CALYPSO while obtaining the same crystal structure.energy cost.（2）Using CRYSTREE to perform structure search prediction under high pressure on Li2SiH6,three new structures were obtained.Li2SiH6 undergoes two high-pressure phase transitions between 0 and 400 GPa,at 21 GPa and 91 GPa,respectively,from the triangularly symmetric P312phase and the phase to the hexagonally symmetric phase under high pressure.Under high pressure,the cell volume of Li₂SiH₆is continuously compressed,and the atomic spacing is continuously reduced.Using WIEN2K to calculate the electronic energy bands and electronic density of states of its three phases,it is concluded that with the increase of pressure,Li₂SiH₆transforms from an insulator under normal pressure to a weak metallicity to finally metallicity.（3）This paper further carried out research on the superconductivity of Li₂SiH₆,and found that applying pressure is beneficial to increase its electron-phonon coupling constant,which leads to its superconductivity.Electron-phonon coupling calculations show that it is superconductive after 200GPa and has a maximum superconducting transition temperature of 56 K.Although the superconducting transition temperature of this ternary metal hydride is not very high,it is hoped that it can provide some reference for the research of other ternary hydrides under high pressure.