Theoretical Study Of High Pressure Phase Transition And Superconducting Phase Transition Based On Electron Density Of States

In the theoretical study of high pressure phase transition,the most commonly used criterion is the classical thermodynamic criterion.Tracing back to the nature of phase transition,the parent phase always transforms to a new phase with lower Gibbs free energy.The thermodynamic criterion of phase transition predicts whether the phase transition will occur according to whether the nth derivative of the state parameter is discontinuous.However,the continuity verification of multi-order derivatives increases the complexity of judgment to a certain extent.In the high-pressure phase transition,when the temperature is constant,the solution of Gibbs free energy is mainly focused on the calculation of enthalpy.When the enthalpy values of the two phases intersect with increasing pressure,the intersection point is considered to be the phase transition pressure point.In this study,the author tries to introduce a new perspective in the judgment of high pressure phase transition,which is the electron density of states(DOS).First,a series of transition metal dichalcogenides(TMDs)are selected as the research samples,and first-principles calculations based on density functional theory are employed to calculate the enthalpy values and DOS of the 2H_c-and 2H_a-phases of six TMDs under different pressure conditions.Mo S₂,Cr S₂ and WS₂ show enthalpy intersection points at 16.1 GPa,42.2 GPa and 76.7 GPa,respectively.For the DOS analysis near Fermi level of the 2H_a phase of these three materials,the abnormal deviation of the DOS curves was observed at these pressure points.Therefore,a new criterion named Conch Criterion,is proposed to judge phase transition based on DOS.The new criterion can observe the occurrence of phase transition from another perspective while mutually confirming the classical thermodynamic criterion.In addition,Conch Criterion is different from other phase transition criteria in that the pressure point can be traced only by the physical properties of the new phase.In this thesis,utilizing this characteristic,we calculate the DOS of HP2-Cu2S,B2-Pb S,B2-Pb Se and B2-Pb Te under different pressures.The commonality of these crystals lies in the fact that the crystal structure data of their parent phases are unknown.Under such a premise,Conch Criterion successfully verified the phase transition points measured in the experiment only according to the DOS curves of the new phase.Finally,the DOS perspective is focused on the superconducting phase transition,which is closely related to high pressure.Taking Hg and Zr Te₃,two typical superconducting materials as research samples,first-principles molecular dynamics simulations are used to calculate their DOS versus temperature under different pressures.Compared with other temperature ranges,the results show that DOS exhibits more obvious fluctuations near the superconducting transition temperature T_c.Thus,further analysis of the PDOS was carried out,and the main inductive factors leading to the superconducting phase transition of Hg and Zr Te₃ were successfully captured.At the same time,through the study of the atomic motion path of Zr Te₃ versus temperature under 10 GPa and 20 GPa,the microscopic mechanism of pressure effectively increasing the T_c is deduced.