First Principles Calculations On Several Minerals Under High Pressures

In this study,potential Fe halides at different pressures inside the Earth were investigated.The existence of stable Fe-halides at extreme pressure plays a key role on the understanding of missing heavy halogen paradox.The results show that during the initial stage of the Earth's accretion,the pressure inside the earth was only several tens of GPa when heavy halogens could react with iron to form stable halogen-rich iron halides before the formation of the earth's core.With the accretion of the earth and the formation of the core,part of these halogen may have entered into the core.Due to the increase of pressure,Fe halides inside the core would release halogens continually to form more stable iron-rich halides.Thus the stability of Fe halides may have significant influence on the volatility of halogen.The calculations also revealed a novel phase,Fe₂I.In this compound,the I atoms are positively charged by donating some electrons to iron!In Fe₂I,half of the iron and I atoms formed polyhedra,while the remaining half number formed one-dimension Fe chains intercalated in the open channels.First-principles calculations were used to study the high-pressure structure of glassy SiO₂ and GeO₂ as well as the relationship between coordination number and oxgey packing fraction in SiO₂ and GeO₂ glasses and crystals were examined.Our calculations show that the coordination number of A atom in a A-O glass changes continually with pressure,unlike in the corresponding crystalline phases where the coordination number changes abruptly at the transition.Moreover,at ultra-high pressures,the spherical electron distribution of the electron of an atom may be distorted.Thus,the traditionally used rigid-ball model and bond length relationship may not be appropriate.Quantum mechanics method is needed to test the validity of the over-simplified rigid ball packing model on the description of the structural changes in glasses at high pressure.First-principles calculations were used to study the structures and viscosity of CaCO₃ melt at high pressures.The calculations reveal that when pressure is below11.2 GPa,CaCO₃ melt behaves like an ideal liquid with extremely low viscosity,close to that of water,consistent with the experimental observation;while at higher pressures,the calculated viscosity increase quickly.It was found that when pressure was raised above 11.2 GPa,the distance between CO₃^2-ions decreased and forming temporal CO₃^2-clusters.At the same time,the correlation between Ca and C,as well as Ca and O has increased.The calculated viscosity,reproduced the experimental viscosity at low pressure and,in addition,revealed a discontinuity at high pressures.Therefore,one cannot extrapolate properties measured at low pressures to high pressures.The content and distribution of light elements in the Earth's core has been the focus of scientists,and has a highly relevance to the viscosity,electrical and thermal conductivity of possible Fe alloys in the outer core.S is one of the candidates.In this section,first-principles calculations were used to study the structure,density,bulk modulus,velocity and viscosity of Fe-S alloy with different S content during the pressure range of the Earth's outer core and at repectively 7000 K and 8000 K.We find the trend in the sound velocities of Fe-S alloy liquid shows an anomaly to the S content.At 7000 K,the sound velocity of Fe-S alloy decreases with the increase of S content,while at 8000 K,the reverse trend is observed.Furthermore,the calculations show that the S content has little effect on the viscosity of Fe-S alloys.A further study is needed to explore how S affect the properties of Fe-S alloy,so that we can determine the S content in the earth's outer core.