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Title: Ab initio lattice dynamics and structural phase transitions
Author: Warren, Michele Carol
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1997
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Prediction of the conditions required for the transformation of one phase of a mineral into another has long been a goal of condensed matter physics. This is especially desirable for phase transitions which are believed to be involved in geological processes, but for which the conditions of temperature or pressure are hard to reproduce experimentally. This thesis examines a number of structural phase transitions including those of MgSiO3 perovskite, which is thought to form the largest part of the Earth's mantle and of ZrO2 which plays an important role in inhibiting crack formation in ceramics. These phase transitions, in which an alternative phase may be reached by continuous distortions of the structure on an atomic level, are examined primarily through the use of first principles electronic structure calculations. Existing first principles techniques were extended to facilitate determination of the equilibrium structure by relaxation of the unit cell and the calculation of the lattice dynamics of complex phases. The distortion involved in most of the phase transitions studied is found to reflect the normal vibrational modes of one or both phases. The phase transitions of MgSiO3 are found to be well described by only a few normal modes of the highest-symmetry cubic phase, dominated by two modes involving tilting of the SiO6 octahedra. These modes resemble rigid unit modes, in which SiO6 octahedra are assumed to remain perfectly rigid but may rotate with respect to other octahedra, whilst preserving linkages between them. The extent to which such simple modes are an accurate description of the dynamics of MgSiO3, BaZrO3 and SiO2 is investigated by way of structural analysis and lattice dynamics of both stable and metastable phases. Both simple models deduced from the lattice dynamical analysis and molecular dynamics using forces calculated from first principles are used to estimate transition temperatures for thermally induced phase transitions in MgSiO3.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available