First-principles Investigations Of Crystal Structures And Physical Properties Of Jadeite And Kosmochlor Under High Pressures

Pyroxene is the main component of mantle which is often used as a trace of the depth of the earth.Jadeite(Jd)and kosmochlor(Kos)are the main components of Na element in the mantle.The structure and phase transition information of jadeite and kosmochlor under high pressure is important to the understanding of complete mantle mineralogical model.It also can predict the occurrence form of Na element and explain the origin of jadeite in ultrahigh pressure(UHP)metamorphic rocks.However,there are many limitations on high pressure experimental conditions.With the development of quantum mechanics and the increase of computing power,the application of the first-principles can be used to verify the experimental data and compensate for the defects of the static high-pressure technology or the impact high-pressure technology.This study based on the plane wave pseudopotential density functional theory using the first-principle calculation to calculate the physical properties of jadeite and kosmochlor,including lattice parameters,elastic constants,electronic and optical properties.The lattice parameters calculated by GGA-PBEsol at high pressure agrees well with the experimental results,the error is within 0.15%,which is superior to that of kosmochlor.The unit cell volume of kosmochlor calculated under the same pressure is larger than that of jadeite because of the atomic radius of chromium ions is greater than that of aluminum ions in octahedral M1 position.The bulk modulus of jadeite under high pressures is always greater than that of kosmochlor.The bond length b changes of jadeite and kosmochlor are almost the same,so are the length c changes,while the bond length a changes of kosmochlor is greater than that of jadeite because of the different domination of the M1 atoms.The change rate of length b of the two pyroxenes is about 1.3 times larger than that of length c.The bulk modulus of jadeite is greater than shear modulus at the same pressure,which means the compressive capacity is always greater than the shear capacity.Pressure increases the ductility of jadeite,the Vickers hardness formula is more convincing than.The bulk modulus,shear modulus and Young's modulus of kosmochlor increase with pressure,but G/B value and Poisson's ratio reach a maximum at the pressure of 20 GPa.After calculating the crystal structure and verify the mechanical stability criteria,jadeite undergoes a phase transition above 60 GPa.There is no phase transition(0-40 GPa)in the study of kosmochlor although it has anomalous lattice parameters changes in 30 GPa.The phase transition pressures of the two pyroxenes are greater than the pressure of maximum subduction depth of the oceanic crust,so they can be used as a trace of the earth's depth change.In this situation,Jadeite and kosmochlor are still the main forms of Na in upper mantle.In electrical analysis,the energy band structures show that jadeite is a direct bandgap semiconductor.The pressure increase causes the total density state(DOS)of jadeite and kosmochlor below Fermi level drifting towards low energy;the DOS above Fermi level drifts towards high energy which increase the energy band gap and the insulativity of jadeite and also increase the metallicity of kosmochlor.The DOS of jadeite change more rapidly than kosmochlor with pressure,reflecting there are more valence bond overlap and orbital hybridization of Si-O bonds than Cr-O bonds,resulting the structure strength of aluminum-oxygen octahedral structure is stronger than that of chromo-oxygen octahedron under high pressure.The pressure resistance of jadeite is higher than that of kosmochlor under high pressure on macro behavior.The optical analysis showed that the real and imaginary parts of the dielectric functions of jadeite and kosmochlor drifted toward high energy with peak values increased as pressure increased.The changes of the real part of the dielectric function under high pressure are reflected in the refractive index while the changes of imaginary part are reflected in absorption.The jadeite reflectivity spectrum,absorption spectrum and refractive index spectrum are all include in ultraviolet,while the kosmochlor spectrum range includes ultraviolet,visible light and infrared.The increasement in pressure makes the reflectivity curves,absorption curves and refractive index curves of jadeite and kosmochlor moving towards high energy direction with peak values increased.The jadeite absorption edge changes more intense than that of kosmochlor,reflecting the covalency of jadeite is greater than that of kosmochlor.