Theoretical Study of Pressure-Induced Phase Transitions and Thermal Properties for Main-Group Oxides and Nitrides
Type of Degreedissertation
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Main group nitrides and oxides are important solid compounds with applications in fields ranging from structural ceramics to catalysts and electronic materials. We have theoretically investigated the pressure-induced phase transitions and thermal properties for a series of oxides, nitrides and oxynitrides of IIIB, IVB and IIIB group, including Al2O3, AlN, Si3N4, Ga2O3, and Ga3O3N. In this dissertation, thermodynamic potentials at finite temperatures were calculated within the quasi-harmonic approximation (QHA). Structural optimization and total energy calculation of unit cell models were carried out based on first-principles density functional theory (DFT) within the local density approximation (LDA). Vibrational spectra were calculated using the real-space supercell force-constant (SC-FC) method. For pressure-induced phase transitions, we have studied the equilibrium transition conditions, as well as the kinetic process, including the investigation of transition pathways, kinetic barriers and the softening-phonon induced displacive transitions. In our studies of equilibrium transition conditions, we calculated the T-P phase diagrams for Al2O3, AlN, Si3N4, and Ga2O3. Our predicted transition pressures are 84 and 134 GPa at 300 K for corundum→Rh2O3(II)→postperovskite transitions in Al2O3, 9.9 GPa at 0 K for wurtzite→rocksalt transition in AlN, 7.5/7.0 GPa at 300 K for β/α→γ transitions in Si3N4 and 0.5 and 39 GPa at 300 K for β→α→Rh2O3(II) transitions in Ga2O3. In our studies of the pressure-induced reconstructive phase transition pathways, we examined the corundum-to-Rh2O3(II) transition in Al2O3 and the wurtzite-to-rocksalt transition in AlN. We showed that the rhombohedral corundum phase and the orthorhombic Rh2O3(II) phase are related by intermediate structures with monoclinic symmetry (P2/c). Using the proposed transition pathway, we calculated the kinetic barriers for the forward (C-to-R) and backward (R-to-C) transitions and further predict the meta-stabilities of the two phases. For AlN, different transition pathways were previously proposed. We reinterpreted the bond-preserving paths with long-range patterns of the ``transition units'', and our calculated kinetic barriers indicate that the long-range pattern is less important. We also showed that the bond-breaking paths are not energetically favored. In addition, based on the pressure dependencies of the barrier heights we explained the discrepancy of transition pressure between the room temperature observation and the calculated equilibrium result. In our studies of displacive transitions in Si3N4, although β phase is dynamically stable at low pressure, two competing phonon-softening mechanisms are found under high pressure. If the β→γ transition is bypassed due to kinetic reasons at lower temperatures, the β phase is predicted to undergo a first-order β→P3 transition at about 38.5 GPa. This predicted metastable high-pressure P3 phase is structurally related to β-Si3N4. We have also calculated the thermodynamic and elastic properties of these systems, and selected results are presented. Our predictions are in good agreement with available experimental and other theoretical data. Furthermore, we studied the shifted Raman scattering and its correlation with the growth direction in Ga2O3 nanowires. And collaborated with an experimental study that synthesized spinel-structured gallium oxynitride from Ga2O3+GaN mixtures at high pressure and high temperature, we showed that the optimal synthesis pressure is predicted to be close to the β-to-α transition pressure of Ga2O3.
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