First-principles Simulation Of NaCl And Garnet Properties Under High Pressure

Following the development of computer technology,first-principles calculations based on the density functional theory(DFT)have been success-fully used to simulate mineral properties.It has achieved fruitful results,and it has become an indispensable and important tool for high-pressure research.Sodium chloride(NaCl)is one of the most widely used pressure calibration and pressure transmitting medium in high-pressure experiments therefor its stability and structural characteristics under high pressure directly affect experimental results.Garnet is the main component of the lower crust,upper mantle,and transition zone,and its physical properties are of great significance to understand the composition,structure and dynamic process of mantle.In this paper,the first-principles method is used to calculate the effects of hydrostatic and non-hydrostatic pressure on the structure and elasticity of NaCl,and the structure,elasticity and seismic wave velocity of six common garnets under high pressureThe main conclusions are as follows:1.There is a certain deviation between the non-hydrostatic pressure and the NaCl lattice constant under hydrostatic pressure,and the non-hydrostatic pressure breaks the cubic symmetry of NaCl.The NaCl lattice constants of the B1 and B2 phases decrease nonlinearly with increasing pressure.The density under hydrostatic pressure is same as the density under non-hydrostatic pressure.So the NaCl under non-hydrostatic pressure can be used as a pressure calibration to determine the pressure,but it may not fully reflect the stress state during the high pressure experiment.One of the reasons for the difference between the simulation study and the experimental results may be that the ideal hydrostatic pressure state cannot be maintained during the high-pressure experiment2.Under differental stress conditions,the bulk elastic modulus of B2 phase NaCl are larger than that of B1 phase,however the shear modulus are lower than that of B1 phase.Therefore,when the B1 phase is transformed into the B2 phase,the bulk elastic modulus increases and the shear modulus decreases.Non-hydrostatic pressure has different effects on NaCl Young's modulus in different crystal directions.3.The non-hydrostatic pressure will not change the B1 to B2 of NaCl phase transition pressure4.The lattice parameter of 6 common garnets(pyrope,almandine,spessartine,uvarovite,grossular,andradite)increases with pressure decrease.In the research pressure range,the lattice parameter of pyralspite(pyrope,almandine,spessartine)is smaller than that of ugrandite(uvarovite,grossular,andradite),the lattice parameter is mainly affected by the ion radius.The density of almandine,spessartine,andradite is higher than that of the Earth density model,and the pyrope,grossular,uvarovite intersect the earth density model at around 410 km5.The volume of[SiO4]tetrahedron,[YO6]octahedron and[XO8]dodecahedron of garnet decreases linearly with increasing pressure,and the change ratio is close to 1:2:3,indicating that the compression of garnet is mainly controlled by dodecahedron.The results of bond angle variance show that high pressure makes the tetragonal and octahedral of ugrandite more regular,but the distortion of pyralspite tetrahedron increases6.The bulk modulus of the garnet increases nearly linearly with increasing pressure Overall,the shear modulus will increase with the increase of the content of uvarovite and grossular,and decrease with the increase of the content of almandine and andradite,this trend is exactly the same as the change of bulk elastic modulus.Wave velocity results show that the substitution of Cr3+ and Al3+for Fe3+on the octahedral of andradite will lead to the increase of P-wave and S-wave velocity.The existence of garnets and solid melts with different compositions has an important influence on the wave velocity structure of the Earth's mantle.