The Electronic-Structure And Optical Properties Of Ferropericlase,Perovskite And Postperovskite At Pressure Conditions Of The Earth's Lower Mantle

The?Mg,Fe?O ferropericlase and?Mg,Fe?SiO₃ perovskite are important minerals in the Earth's lower mantle,and its post-perovskite is a major material in the D”region of the lower mantle,It is of great importance in Earth science to explore the physical properties of these materials at pressure conditions of the Earth's lower mantle.Recently,The thermal radiation effect of lower mantle minerals have become a significant issue in the high-pressure science,because these information is of critical role for understanding lower-mantle thermal structure,evolution,and dynamics.In order to obtain them,the high-pressure optical-absorption spectrum and refractive-index data of the?Mg,Fe?O ferropericlase and?Mg,Fe?SiO₃ perovskite and post-perovskite are needed.Because optical properties of these minerals are closely related to their electronic structures,this obtained information is helpful for understanding the microscopic mechanism of the change of their optical properties.In this thesis,the electronic structure and optical properties of?Mg,Fe?O ferropericlase and?Mg,Fe?SiO₃ perovskite and post-perovskite without and with O ionic vacancy at high pressure have been studied by the first-principles calculation based on density functional theory.The main research work and conclusions of this paper are as follows:1 Using the first-principles calculations,the electronic structures and optical properties of(Mg_0.8125,Fe_0.1875)O ferropericlase crystals without and with O2-vacancy under the pressure of the Earth's lower mantle are investigated.The calculation results show that similar to predictions from the crystal-field theory,the electronic spin transition of ferrous in perfect-crystal causes a wider band gap and an obvious blue-shift in its optical-absorption spectrum,becoming transparent below the wavenumbers of ¹5000cm-l.However,the optical absorption in the near-infrared region of defective crystal is significantly enhancedwith the spin transition of iron impurity.That means,in a real doping concentration,the pressure-induced O ionic vacancy point-defect may be a important reason to cause the essential differences between high-pressure absorption spectrum experiment and crystal-field theory predicts.The refractive-index data of perfect-crystal indicate that: the spin transition of iron has an unobvious influence on its refractive index change along with the wave number,faintly rising with the increase of pressure.However,when there are O2-point-defect in(Mg_0.8125,Fe_0.1875)O ferropericlase,Pressure,wave number and the spin transition of iron are factors to cause its high refractive index change significantly.2 The calculated data of electronic structures show that the effects of O ionic vacancy and spin transition have made a blueshift in total density of states and the larger width in band gap.The Valence band and Conduction band become narrow and their peak intensity increases,the number of main peak reduces.These changes result in the enhancement of light absorption,which is similar to the observation in the high-pressure absorption experiments.3 The optical properties of(Mg_0.875,Fe_0.125)SiO₃ perovskite and?Mg0.9,Fe0.1?SiO₃post-perovskite at high pressure have been calculated by the first-principles ultra-soft pseudopotential approach of the plane wave.The calculated data show that: The structural transition in perovskite causes an increase in the absorption,confirming the inference from experimental data.The wave-number peak of the ferrous-iron absorption band in post-perovskite is close to experimental observations.In the post-perovskite region,the absorption-band intensity is slightly enhanced with increasing pressure,but the spin transition in ferrous-iron could have little influence on the absorption spectrum.The perovskite-post perovskite structural transition results in a refractive-index increase;in the post-perovskite region,the effects of the pressure and spin transition are not obvious.4 The calculated data of electronic structures show that the effects of the pressure on the density of states in different structural phases are unlike.In the perovskite region,the density of state is varied slightly with increasing pressure.However,in the post-perovskite region,the Peak intensity of density of states decreases with pressure?the peaks in density of states blueshift significantly?,and the number of peaks reduces?the width in peaks broaden?.The changes are consistent with the ones of the calculated optical properties.