Prediction Of The High-pressure Melting Phase Diagrams Of The Lower Mantle Minerals FeOOH And MgSiO₃

The study of the Earth and Earth-like planets can help us understand the origin of natural disasters such as earthquakes,volcanic eruptions,and tsunamis,and greatly reduce or even prevent the risk to life and property caused by these disasters,as well as provide information resources for geomining,which can bring great economic benefits.However,the technology currently available to people cannot directly study the Earth’s interior,but it can gradually complement and improve people’s understanding of the origin,internal structure,dynamical processes,and chemical composition of the Earth in an indirect way by calculating the properties of minerals in the Earth’s interior.This thesis presents a comprehensive and systematic study of the structural,mechanical,thermodynamic and melting properties of FeOOH and MgSiO₃ using methods based on first-principles calculations and classical molecular dynamics simulations.The phase transition sequence of the structure of FeOOH in the lower mantle pressure range（0-136 GPa）was determined in the first part of this thesis by means of the first-principles calculations.α-FeOOH with Pnma space group changed toε-FeOOH with Pmn2₁structure at 4.9 GPa,and the hydrogen bonding symmetry led to the transformation ofε-FeOOH to Pnnm structure when the pressure reached 40.3 GPa.At 69.0 GPa,the Fe atom undergoes a high and low spin transition,resulting in an isostructural phase transition ofε-FeOOH with a significant decrease in volume but no change in space group.After 81.0 GPa,ε-FeOOH transforms into a high-pressure pyrite-FeOOH structure,and the new structure can be maintained until it exceeds the lower mantle pressure,and then the phase transition sequence is analyzed based on the determination of the differences in elastic properties between different structures of FeOOH were then analyzed based on the determination of the phase transition sequence.In order to investigate the thermodynamic properties of FeOOH,a deep learning method was adopted to construct a potential function model applicable to the multiphase structure of FeOOH,The new potential function model can accurately describe the variation of H-O bond length in FeOOH with temperature,and with the help of this high-precision potential function model,the change of secondary phase transition pressure inε-FeOOH caused by hydrogen bond symmetrization with temperature is obtained,which clarifies the disagreement on the pressure of hydrogen bond symmetrization in FeOOH for many years.The results show that hydrogen bond symmetrization does not occur at lower pressures,but deviates from the original equilibrium position due to the large vibrations of H atoms caused by temperature,producing a hydrogen bond disorder phenomenon similar to hydrogen bond symmetrization,which has a variation of about 25 GPa in the temperature range of 0-300 K.In addition,this thesis calculated the bulk compression and thermal expansion properties of FeOOH in the pressure range of the lower mantle,and determined the decomposition temperature of the multiphase structure of FeOOH at high pressure by single-phase heating,and finally the P-T phase diagram of FeOOH in the pressure range of the lower mantle was plotted collectively with the results of first-principles calculations.In the second part of this thesis,the pressure of the transition from the perovskite to the post-perovskite structure of MgSiO₃ was determined by the first principles calculation method to be 109.6 GPa.The higher phase transition pressure indicates that the post-perovskite structure may be abundant in the D’’layer at the bottom of the lower mantle.The calculations of the elastic properties of MgSiO₃ show that the phase transition from the perovskite to the post-perovskite structure does not significantly improve the ability of MgSiO₃ to resist external deformation.The empirical Buckingham potential function for MgSiO₃ under high-pressure conditions is constructed by empirically fitting the elastic properties and crystal structure,and the calculations are also verified under extreme conditions of high temperature and high pressure.By using classical molecular dynamics simulations based on the new potential function,the bulk compression and thermal expansion properties of the perovskite and post-perovskite structures of MgSiO₃ in the lower mantle pressure range are obtained,and the results are in good agreement with the existing experimental or computer simulation results.Finally,the melting curves of perovskite and post-perovskite structures of MgSiO₃ in the pressure range of the lower mantle were calculated by the accurate two-phase coexistence method,and the melting temperature of MgSiO₃ at zero pressure is 2815 K.When the pressure reaches 234 GPa and the temperature increases to 7243 K,the melting curves of perovskite and post-perovskite structures of MgSiO₃ intersect,and the point is the solid-solid-liquid three-phase coexistence point of MgSiO₃.The above conclusion also indicates that the solid-solid phase transition boundary of MgSiO₃ has a positive Clausius-Claperon slope.