First-Principles Study Of FeO₂ And Its Helium-Containing Compounds At High Pressure

The material composition and physical and chemical properties of the earth’s interior are the basis for our understanding of the formation and evolution of the earth.Due to the limitation of borehole sampling and high-temperature and high-pressure experiments,the first-principles simulation method based on quantum mechanics has become one of the important auxiliary methods for the study of the physical properties of the earth’s internal materials.It is the link and bridge between seismological observation and the study of the earth’s internal structure,material composition,and physical and chemical state.The Fe-O system inside the earth undergoes physical and chemical transformations under high temperature and high pressure.The recently discovered FeO₂ appeared in a new stoichiometric ratio and stably exists at the core-mantle boundary,and under mantle pressure,it can form the first stable helium-containing compound with He element.The systematic and comprehensive study of the structure,electronic and elastic properties of FeO₂ and FeO₂He under high pressure is of great significance for understanding the composition of the earth’s internal matter and the physical behavior of FeO₂ and FeO₂He under high pressure,and it also provides evidence for finding He reservoirs inside the earth.In this work,the crystal structure,electronic properties and elastic properties of FeO₂ and FeO₂He under high pressure are calculated by first principles.Through comparison with the PREM model,analysis and discussion of the occurrence characteristics of FeO₂ and FeO₂He in the earth’s interior.FeO₂ and FeO₂He have a pyrite-type cubic crystal structure that can stably exist within the core-mantle boundary.The addition of He increases the stability of the system,while the effect of pressure is opposite.The lattice constant and volume of FeO₂and FeO₂He crystals decrease with the increase of pressure.The addition of He reduces the density of FeO₂.The O-O bond length,Fe-O bond length,and O-Fe-O bond angle in FeO₂ crystals decrease with the increase of pressure,while the Fe-O-O bond angle changes in the opposite direction.On the contrary,the bond angles of O-Fe-O and Fe-O-Fe in Fe O2He do not change with pressure.The addition of He occupies the position of Fe in FeO₂,which breaks the O-O bond,but forms a covalent bond of He and has a larger Fe-O bond.Pressure has little effect on the energy band,density of states and electron density of FeO₂ and FeO₂He.FeO₂ exhibits metallic properties under pressure,and the addition of He makes FeO₂ change from metal to semiconductor,and pressure will significantly increase the band gap of the semiconductor.The conduction mechanism of FeO₂ and FeO₂He near the Fermi level are formed by the electronic hybridization of the d orbital of Fe atoms and the p orbital of O atoms,and the pressure is not conducive to the conduction of the system.The interaction between Fe atoms and O atoms in FeO₂weakens with the increase of pressure,but the interaction between FeO₂ groups and He atoms is basically unaffected.In the Fe-O system,Fe atoms lose electrons and O atoms gain electrons and behave as ionic bonds.The addition of He has no effect on the gains and losses of Fe and O atoms.He has a weak ability to lose electrons and may act as a Coulomb shield in the system.The elastic constants,elastic modulus,Poisson’s ratio,Pugh ratio and wave velocity of FeO₂ and FeO₂He increase with the increase of pressure.The addition of He obviously increases the wave velocity of FeO₂.FeO₂ and FeO₂He have increased resistance to elastic deformation and ductility with the increase of pressure and tend to be isotropic,but they are brittle under low pressure,while stiffness and resistance to shear deformation of FeO₂ are reduced under excessively high pressure.The addition of He improves the ability of FeO₂ to resist elastic deformation,but reduces the original ductility.The water carried by the subducting slab reacts with the iron in the core to form FeO₂,which can be directly present at the core-mantle boundary and become a material composition in the ultra-low velocity zone.FeO₂ moves up under the action of the overlying slab and reacts with the He reservoir to form FeO₂He,which occurs between the top of the core-mantle boundary and the bottom of the lower mantle.It can migrate to the mantle transition zone or become a material composition of ultra-low velocity zone through the action of the subduction zone.But FeO₂He may be more located at the top of the ultra-low velocity zone.On the contrary,the stable existence of FeO₂He proves that there should be He reservoirs in the core-mantle boundary or the lower mantle.