Atomistic Simulation of Barium Titanate

Citation

Zhang, Qingsong (2005) Atomistic Simulation of Barium Titanate. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SQ9J-4H73. https://resolver.caltech.edu/CaltechETD:etd-10292004-152709

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° domain wall is Ba-centered with abrupt polarization switching across the wall. The center of BaTiO₃ 90° 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° 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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