First-principles Thermodynamics of Phase Transition: from Metal to Oxide
Open Access
- Author:
- Mei, Zhigang
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 14, 2011
- Committee Members:
- Zi Kui Liu, Dissertation Advisor/Co-Advisor
Long Qing Chen, Committee Member
Vincent Henry Crespi, Committee Member
Clive A Randall, Committee Member - Keywords:
- First-principles
thermodynamics
phase transitions
Ti
TiO2
BiFeO3 - Abstract:
- Many interesting phenomena in physics, geosciences, and materials science are directly related to phase transitions, such as ferroelectricity, ferromagnetism, and multiferroicity, which can be induced by factors such as temperature, pressure, and strain. Various aspects of phase transitions have been studied such as thermodynamics, kinetics, and crystallography. The present dissertation focuses on thermodynamics which dictates not only equilibrium states of materials, but also the driving force for phase transitions. With the advancement of density functional theory and computing technology, quantum mechanical first-principles calculations have become a powerful tool for materials design. In this work, density functional theory based first-principles calculations coupled with ab initio molecular dynamics and CALPHAD (CALculation of PHase Diagram) methods are applied to study thermodynamics of phase transitions and aid materials design in three systems from elemental metal to complex oxides: Ti, TiO2, and BiFeO3. Ti and Ti-based alloys are widely used in numerous applications due to its exceptional strength-to-weight ratio, high temperature performance, and corrosion resistance. The mechanical properties of Ti alloys can be greatly improved by controlling the crystal structures present. Pressure is a very important variable in causing phase transitions in this element, and some of the pressure-induced phases can be retained in a metastable form after removal of pressure. In this work, the pressure-induced phase transitions and equilibrium pressure-temperature phase diagrams of Ti are studied using first-principles calculations. The phase stabilities of γ- and δ-Ti phases under hydrostatic compression are clarified from a systematic study of pressures using various equation-of-state fittings and direct first-principles calculations. Due to the dynamical instability of β-Ti at low-temperature, ab intio molecular dynamics simulations together with thermodynamic integration are used to explore the hcp/bcc free energy difference and the phase transition along Burgers paths. The thermal electronic entropy is found to play a critical role in the stabilization of bcc Ti at high-temperature. TiO2 possesses a rich phase diagram with many polymorphs. The recently discovered high-pressure cotunnite and cubic phases of TiO2 show great promises for ultra-hard materials and future-generation solar cells, due to their large bulk modulus and excellent optical properties, respectively. Their phase stabilities at ambient conditions are, however, not clear. In this work, a complete study of the phase stabilities of all TiO2 polymorphs is provided. The calculated phonons show that all high-pressure phases, except the cubic phase, are dynamically stable at ambient pressure, indicating cotunnite phase might be quenched to ambient conditions. Meanwhile, it is predicted that fluorite TiO2 is dynamically stable at high pressures and can be assigned as the synthesized cubic phase. The equilibrium pressure-temperature phase diagram of TiO2 is determined from the calculated Gibbs energies, which provides useful information to the synthesis of the high-pressure phases of TiO2. Multiferroic material BiFeO3 is a promising material for lead-free piezoelectric applications. High quality multiferroic BiFeO3 films are, however, difficult to obtain due to its small processing window. With coupled first-principles calculations and CALPHAD method, the growth conditions for BiFeO3 thin films are explored by thermodynamic calculations. The formation enthalpy of BiFeO3 predicted by first-principles calculations is used to study the phase equilibrium and chemical potential-temperature phase diagram. The chemical potential of Bi is predicted to be useful in tuning the stability window and tailoring the processing conditions of BiFeO3. Furthermore, the effect of epitaxial strain on the structure and properties of BiFeO3 is studied by first-principles calculations to understand strain-induced morphotropic phase boundary in BiFeO3. A first-order rhombohedral-like to tetragonal-like phase transition is predicted at a critical compressive strain of 5.15%, and this isosymmetric phase transition enables the coexistence of these two phases and the morphotropic phase boundary behavior reported in the strained BiFeO3 films. This work provides an in-depth study of the mechanisms of the pressure-, temperature- and strain-induced phase transitions in three selected materials. An improved understanding of the thermodynamics of phase transitions and the conditions of phase stability would help to further optimize the properties and improve the performances of these materials.