First-principles Investigations On High Pressure Propertites Of Fe-bearing MgO

The crystal and electronic structures of materials under high pressure would have a significant change and even show an exotic behavior which distinct from that in the ambient condition.Therefore,the high pressure,as a useful tool,are widely applied to lots of material research.The Earth's lower mantle(LM)with a depth of 660-2900 km and a pressure range of 25?120 GPa,as the largest continuous area in Earth,accounts for almost two thirds volume and half mass of the Earth.Therefore,the study of the main material components in LM plays a critical role in understanding the internal structure and dynamic properties of the Earth.The determination of the chemical composition and the elemental distribution of the material within LM is a long-standing challenge for Earth science.Ferropericlase(Fp)is the second abound mineral in the Earth's LM.The distribution and spin transition of iron,and the defect concentration of Fp have important influence on many properties of Fp.Our studies would provide some useful information for understanding the behavior of iron in Fp and are of great significance to geoscience.The mainly researches of this thesis are described as follows:(1)Firstly,we investigate the distribution of Fe in a large supercell containing more than 93000 atoms of Fp by using cluster expansion theory and Monte Carlo simulations.Combined the distribution of Fe in Fp with the linear fitting of the experimental results,we get the spin transition zone of Fp.We find that Fe distribution can induce a broad transition zone comparable to the experimental observations,which gives a reasonable explanation of the controversy of experimental and theoretical results.These results are consistent with the smoothly changing properties of lower mantle as well.(2)The defect Fe-bearing MgO is investigated based on the DFT calculations.We find that Fe prefers to substitute the neighbor site of Mg vacancy with a notable change of Fe-O bond length(reduced about 5%),and the valence state and magnetic moment of Fe(from +2 to +3).Additionally,iron incorporation can increase the Schottky defect concentration of Fp(increased about 20 times at the starting point of lower mantle),especially in the lower mantle conditions.Which would have a significant effect on the properties of lower mantle.(3)We calculate the migration energy of H in MgO under different pressures using NEB method.This work illuminates that H atoms prefer to combine into H molecular in MgO,and their migration barriers increase as a linear function of pressure.And then,the attempt frequency of H was calculated by DFT-MD method.Finally,we get the migration rate of H in MgO,D(T)?5.86×10-4(cm2/s)exp(-0.44 eV/kBT),through Arrhenius equation,which is much higher(about 3×103 at 1000 K)than that in Al2O3.Besides,these results are also helpful for the understanding of the water transport into the deep mantle.In brief,our computational results illustrate the high pressure properties of Fe-bearing MgO,and can also provide some guidance for experimental measurements,which would arouse people's interest in the study of minerals in Earth's interior.