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Type of Document	Dissertation
Author	Zhang, Qingsong
Author's Email Address	qsz AT wag.caltech.edu
URN	etd-10292004-152709
Persistent URL	http://resolver.caltech.edu/CaltechETD:etd-10292004-152709
Title	Atomistic simulations of barium titanate
Degree	PhD
Option	Materials Science
Advisory Committee	Advisor Name Title Sossina M. Haile Committee Chair David G. Goodwin Committee Member Guruswaminaidu Ravichandran Committee Member Harry Atwater Committee Member Kaushik Bhattacharya Committee Member Michael Ortiz Committee Member Tahir Cagin Committee Member William A. Goddard III Committee Member
Keywords	Barium Titanate Domain Wall Oxygen Vacancy Phase Transition
Date of Defense	2004-09-30
Availability	restricted
Abstract We present the Polarizable Charge Equilibration (P-QEq) force field to include self-consistent atomic polarization and charge transfer in molecular dynamics of materials. The short-range Pauli repulsion effects are described by two body potentials without exclusions. A linear self-consistent field solution to the charge transfer is proposed for charge transfer in large systems. The P-QEq is parameterized for BaTiO3 based on quantum mechanics calculations (DFT with GGA) and applied to the study of the phase transitions, domain walls and oxygen vacancies. Frozen phonon analysis reveals that the three high-temperature BaTiO3 phases in the displacive model are unstable. Within their corresponding macroscopic phase symmetries, the smallest stable phase structures are achieved by antiferroelectric distortions from unstable phonons at the Brillouin zone boundaries. The antiferroelectric distortions soften phonons, reduce zero point energies and increase vibrational entropies. A correct BaTiO3 phase transition sequence and comparable transition temperatures are obtained by free energy calculations. The inelastic coherent scattering functions of these phases agree with X-ray diffraction experiments. BaTiO3 180 degree domain wall is Ba-centered with abrupt polarization switching across the wall. The center of BaTiO3 90 degree domain wall is close to its orthogonal phase. There are transition layers from the wall centers to the internal domains in the types of domain walls. Polarization variation in these transition layers induces polarization charge and free charge transfer. This effect causes a strong bipolar electric field in BaTiO3 90 degree domain wall. Oxygen vacancies are frozen at room temperature, and mobile near the Curie temperature. In the tetragonal phase, the broken Ti-O chains are frozen, reducing switchable polarization. Due to charge redistribution and local relaxation, oxygen vacancy interaction is short-range and anisotropic. Two oxygen vacancies can form a stable pair state, where two broken Ti-O chains are aligned parallel. Oxygen vacancy clusters can form dendritic structures as a result of local relaxation and charge interaction.
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