First-principles Study Of Novel FeO₂-H-He Compounds Under High Temperature And Pressure

The earth is the cradle of people's survival.The resources that people need for production and life are all from the Earth.Therefore,the evolution and composition of the earth have always been the focus of research.The interior of the earth is a high-temperature and high-pressure environment.The pressure and temperature from the crust downward gradually increase.It is mainly divided into three parts:the crust,the mantle,and the core.Furthermore these three parts are composed of many small parts.The inside inner core's pressure is up to³60 GPa and the temperature is up to⁶500 K.The composition of the Earth is very complex.According to current technological level,It is difficult to intuitively detect the actual situation in the Earth with the current technological level.Researchers explore the composition and evolution of the Earth through three ways,high temperature and high pressure theoretical prediction,high temperature and high pressure experimental simulation and seismic wave detection.With the development of computer technology in recent years,first-principles methods and high pressure structure prediction methods have been widely used.It can effectively predict high-pressure structures that are not easily synthesized experimentally.In this paper,based on the research of the CALYPSO structure prediction method and structure prediction software developed by the research group,we explored the existence form of helium in the earth,the possibility of FeO₂H₂He in the earth and the interaction strength of H-He.1.The mass of helium is so light,and the light elements are difficult to keep in the earth during the formation of the Earth.Therefore,except for ⁴He which can be decayed inside the earth,the original ³He should not exist inside the earth.However,through the study of volcanic eruption elements and ⁴He/³He isotope ratios in different places on the earth,it is found that the interior of the earth contains two helium reservoirs.There is a more primitive ³He helium reservoir at the deep of the mantle and the helium element is likely to exist deep inside the Earth.However,there are two electrons outside the helium element core,which is occupied by a 1s orbital full shell,and its chemical properties are very inactive and difficult to react with other elements.How does helium exist inside the earth?Where does it exist?It is a important scientific question.We used the structural prediction software combined with the first-principles software to predict the possible formation of 100-300 GPa of FeO₂ and He,and found a stable high-temperature and high-pressure compound FeO₂He.The stability of FeO₂He at high temperature and high pressure indicating that it may exist at the core-mantle boundary,and its'compression and shear wave speeds are consistent with the ultra-low velocity region at the core-mantle boundary.This shows that FeO₂He is likely to be a candidate substance in the ultra-low velocity region,which adds a new explanation for the formation of the ultra-low velocity region.This study has deepened people's understanding of the ultra-low-velocity regions inside the earth,and increased people's knowledge about the physical and chemical properties of helium.2.Helium is the second most abundant element in the universe is second only to the element of hydrogen.Hydrogen and helium are the main components of the atmosphere of the ice giant planets.Our first research work found that FeO₂He is likely to exist at the boundary of the core and the mantle.Previous studies have also shown that FeO₂,FeO₂H,FeO₂Hx,H,He,etc.also exist inside the earth,so we raised a scientific question can hydrogen and helium react with FeO₂ to form a quaternary Fe-O-H-He compound?The strength of the interaction between hydrogen and helium has always been a hot issue discussed by people.There is a lot of controversy about whether they are bonded or not.Previous studies believed that H and He were bonded,and the Raman vibration peak of H and He bonding was observed.However,in subsequent studies,it was discovered that the Raman fronts of H and He they observed were not bonds of hydrogen and helium,but of nitrogen and helium.There are both H and He elements in the Fe-O-H-He quaternary compound.What is the interaction strength of H and He atoms in this compound?In this paper,we studied the reaction of three elements FeO₂,H and He under high pressure.We found a relatively stable compound FeO₂H₂He.After calculating the mean square displacements at different temperatures,we found that in this compound,with the temperature rises,the H element diffuses first,and then the other element diffusion compounds melt,which is different from the H₂O-He studied previously?in this system,He diffuses before H in the process of heating up?.Through the radial distribution function and the atoms in molecules method?AIM?calculation of intramolecular atoms,we studied the bonding behavior.The distance between the H atom and the He atom of this compound becomes shorter at high temperature and high pressure.The high-temperature and high-pressure stability study shows that the FeO₂H₂He compound is likely to exist in the earth's lower mantle.The P-wave and S-wave velocity and density of FeO₂H₂He are consistent with the ultra-low velocity zone model inside the earth.The comparison of the high-pressure physicochemical properties of the FeO₂H₂He compound with the earth model provides inspiration and knowledge reserves for the circulation of hydrogen and helium inside the earth and the evolution of the earth.