First-principles Calculation On Crystal Structure And Elastic Properties Of Py-FeO₂,Py-FeOOH And ?-FeOOH Under High Pressure

Py-FeO₂,Py-FeOOH and ?-FeOOH are important components of mantle and core-mantle boundary.It is important to understand the composition,structure as well as dynamic process of mantel material based on their physical evolution characteristics at high temperature and high pressure.The crystal structures and elastic properties of Py-FeO₂ and Py-FeOOH at 0-350 GPa pressure and ?-FeOOH at 0-170 GPa pressure were calculated by first principles.Different structural characteristics of the three minerals and hydrogen bond symmetry of ?-FeOOH were analyzed in this paper.The occurrence of FeOOH system in the deep part of the earth as well as its significance to the wave velocity anomaly in the deep part of the earth was discussed in this paper.The structure of Py-FeO₂ and Py-FeOOH is cubic crystal system,the lattice constant decreases gradually with the increase of pressure.The structure of the ?-FeOOH is orthorhombic crystal system,the lattice constant mutates increase at 33 GPa,the mutation of a,b axis length increased,and the mutation of c axis length increased.The cell density of Py-FeO₂,Py-FeOOH and ?-FeOOH increases with pressure,Py-FeO₂>Py-FeOOH>?-FeOOH.The elastic constants C₁₁,C₁₂ and C₄₄ of Py-FeO₂ and Py-FeOOH increase linearly with increasing pressure.The elastic constants C₁₁,C₂₂,C₃₃,C₅₅,C₁₂,C₁₃,C₂₃ of ?-FeOOH increase linearly with the increase of pressure,the elastic constant C₄₄ of ?-FeOOH decreases linearly with the increase of pressure,and the elastic constant C₆₆ of ?-FeOOH has almost no change with the increase of pressure.The elastic constant of ?-FeOOH changes greatly around 33 GPA,which is due to the hydrogen bond symmetry phenomenon of the structure with the change of pressure.This phenomenon shows that the elastic constant of ?-FeOOH is closely related to the strength of hydrogen bond.The bulk modulus of Py-FeO₂,Py-FeOOH and ?-FeOOH increases linearly with pressure,the shear modulus Py-FeO₂ and Py-FeOOH increases linearly with pressure,the shear modulus of ?-FeOOH mutate at 33 GPa.Compared with the bulk modulus,Py-FeO₂ is the highest,and Py-FeOOH and ?-FeOOH is almost the same at high pressure;the shear modulus of Py-FeO₂ is the largest,and the shear modulus of ?-FeOOH is the smallest.The compression wave velocities of Py-FeO₂,Py-FeOOH and ?-FeOOH decrease gradually with the increase of pressure;the shear wave velocities of Py-FeO₂ increase gradually with the increase of pressure.The hydrogen bond symmetry of ?-FeOOH under 33 GPa pressure may affect the crystal cell parameters of ?-FeOOH,the density of the mantle transformation zone,and the seismic wave velocity to a certain extent.The shear wave velocity of Py-FeOOH decreases with the increase of depth in the range of0-2000 km,and changes slightly in the range of 2000-6000km(5.8km/s<Vs<6.0km/s).The shear wave velocities of ?-FeOOH mutate at 33GPa(about 900km),and decreases with depth in the range of 0-900 km,and decreases with depth in the range of 900-3000 km.The wave velocity of?-FeOOH is the lowest,while that of Py-FeO₂ is the highest.Based on comprehensive theoretical calculations,it is found that Py-FeO₂ and Py-FeOOH have the characteristics of high density and low wave velocity,which are consistent with the properties of the mantle ultra-low velocity zone(ULVZs).Py-FeO₂ and Py-FeOOH may enrich and sink to the core-mantle boundary after formation,becoming the source of the ultra-low velocity zone (ULVZs).