First Principle Study On Forsterite Properties Under High Pressure And Temperature

In this thesis, the crystal structure and phase transition behaviors of a andβforsterite (Mg2SiO4) at high temperature and high pressure were investigated by the first principle calculations and the quasi-harmonic Debye model. Variations of elastic properties, band structure, density of state, band gap ofαandβforsterite with pressure were calculated, and the effect ofα/βphase transition on these characters was analyzed. The effects of water on elastic modulus, sound velocity and band gap of forsterite were also simulated. Finally, the effect ofα/βphase transition and water on seismic discontinuity in earth was discussed.The main achievements are as follows:1. Adding water (H-O) in forsterite crystal cell decreases bulk modulus (K), shear modulus (G), and sound velocities (Vp, Vs) of forsterite. The sound velocities of hydrous forsterite reduce by 3.1-7.1% (Vp) and 3.6-9.7% (Vs) respectively at 0-14 GPa with 3.2wt% water. At the same time, the K, G, Vp, and Vs of forsterite decreased too because of density decrease 3.0% caused by H atoms replacing Mg atoms in the crystal. Water also increases the total energy of the forsterite system, i. e. the crystal stability and the power of bond were reduced at high pressure and temperature. This may be the main reason for formation the low velocity layers in the earth.2. The study results of band structure and density of state (DOS) indicate that band gaps and insulativity of a andβphases forsterite increase with increasing pressure. It indicates that electrical conduction ofα- andβ-phases is not caused by transport of protons bounded at Mg tetrahedral position. Conduction of natural olivine is induced by mixing of other particles like Fe, H. When Mg atom holds all cation positions, olivine is not conductive.3. Adding water in the forsterite crystal reduces the bag gap of the crystal. To replace one magnesium atom with two hydrogen atoms increases the electron transport of forsterite crystal, and the hydrous forsterite dose not become a conductor. The substitution of hydrogen with cation of lattice in crystal structure of forsterite has little effect on forsterite conduction.4. Combination of first-principles and the quasi-harmonic Debye model is an efficient approach to simulate the behavior of minerals at high pressure and/or high temperature. Variations of the calculated forsterite properties with temperature and pressure agree well with the experimental data.5. The calculated results are compatible with the previous experimental work at 0-1000 K. The experimental data just fall between LDA and GGA results. The results also show that LDA is more suitable to simulate characters of the olivine mineral system than GGA.6. The average increases of velocities caused by a-|3 phase transition are 11.6% (LDA) and 10.4%(GGA) for Vs and 9.2%(LDA) and 9.0%(GGA) for Vp respectively.