The High Pressure Phase Transition And Structure Stability Of Some Earth Core Elements And Related Systems Based On First-principles Calculations

As one of the basic parameters to change the state of a substance,pressure can change the structure and properties of a material like temperature,so that a phase transition occurs.The studys of high-pressure phase transition not only reveal many special behaviors of the material under high pressure,but also provide important theoretical and experimental basis for high-pressure synthesis.Therefore,the high pressure phase transition becomes an indispensable dimension for exploring new materials.As a high-temperature and high-pressure system,the earth core has attracted much attention.Its main composition is Fe element,which contains a small amount of other metal elements such as:Co,Ni,etc.,and a small number of light elements such as:S.rFle Si,O and so on.Under the extreme conditions,these elements and their constituent alloys must have different structures and properties from the normal temperature and pressure.Although many experiments have studied on this,the detailed high-pressure phase diagrams of many elements have not been plotted,and the structural stability of the core-composite alloys has not yet been determined,due to the lack of experimental conditions.It is necessary to study the high-pressure phase transition of the core material by computational simulation.In this study,the first-principles method based on density functional theory(DFT)was used to study structural phase transition,thermodynamic properties,elastic properties and P-T phase diagrams of core elements(Fe,Co,Ni,and Si)under high temperature and high pressure.The crystal structures of Fe-X(X:Ni,Si)binary alloys with different compositions under extreme conditions were predicted,by lattice dynamic,mechanics and thermodynamic stability.The influence of the composition,pressure and temperature on the stability of the alloy structure was explored.The main research content is as follows:(1)The first-principles method of density functional theory was used to calculate the thermodynamic properties of Fe,Co,Ni and Si at high temperature and high pressure.The solid phase transition points were determined based on the thermodynamic properties at any temperature and pressure.The solid-liquid transition points were calculated using the two-phase model of molecular dynamics and the empirical formula based on elastic constants.By comparison with the existing experimental values,the present pressure-temperature phase diagrams of Fe,Co,and Si and the melting curve of Ni(Ni does not undergo phase transition at high temperatures and pressures)are closed to the experimental values.In addition,the magnetic properties of the three metals under high pressure have also been investigated,to reveal their magnetic properties under high pressure conditions.(2)The structure stability of Fe-X(X:Ni,Si)binary under earth cores condition were investigated from the aspects of lattice dynamics,mechanics and thermodynamic stability.In terms of lattice dynamic stability,for the Fe0.9375X0.0625,the phonon spectra at 300 GPa were calculated for the alloys to investigate the lattice dynamics stability of Fe-Ni and Fe-Si alloys near the earth core pressure;in terms of mechanical stability,the elastic constant of Fe-X alloys at 300 GPa were calculated,according to the mechanical stability criterion,to discuss its mechanical stability under the core pressure;in terms of thermodynamic stability,the free energy of Fe-X alloys were calculated as a function of temperature and pressure to investigate the structural stability of Fe-X alloys at various temperatures and pressures.