Date of Award
Doctor of Philosophy (PhD)
Andrew M. Rappe
Ferroelectric materials exhibiting switchable polarization have been used as critical components in electronics, memory, actuators and acoustics, and electro-optics. The applications of ferroelectric materials heavily rely on the interactions between the polarization and external perturbations, such as electric field, stress, and temperature. It is therefore crucial to understand the dynamics of ferroelectric response at finite temperature. Despite the tremendous advance of computational power and the success of first-principles methods, large-scale simulations of dynamics in oxides at finite temperature can still only be performed using classical atomistic potential. We first develop a model potential based on principles of bond-valence and bond-valence vector conservation. The model potentials for PbTiO3 and BiFeO3 are parameterized using the results from first-principles calculations. The bond-valence-based force field allows for molecular dynamics simulations of ferroelectric response at large time and length scale. The intrinsic inertial response of ferroelectric domain walls is studied in PbTiO3. Examination of the evolution of the polarization and local structures of domain walls reveal that they stop moving immediately after the removal of the electric field, demonstrating that ferroelectric domain walls do not exhibit significant intrinsic inertial response. Taking the 90Â° domain walls in PbTiO3 as an example, we quantitatively estimate the domain wall velocity under a wide range of temperatures and electric fields. We find that many properties of ferroelectrics are dictated by the intrinsic nature of domain walls. We demonstrate that even in the absence of defects the intrinsic temperature- and field-dependence of the wall velocity can be described with a strongly non-linear creep-like region and a power-law depinning-like region. We propose a simple universal nucleation-and-growth-based analytical model that is able to quantify the dynamics of all types of domain walls in various ferroelectrics; this enables the prediction of the temperature- and frequency-dependence of coercive fields at finite temperature from first-principles. We also investigate the orientation-dependent evolution of nanoscale ferroelectric domain structures in PbZr0.2Ti0.8O3 films. Molecular dynamics simulations predict both 180Â° for (001)-/(101)-oriented films and 90Â° multi-step switching for (111)-oriented films, and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. Finally, we investigate the domain walls in organometal halide perovskites. We find that organometal halide perovskites can form both charged and uncharged domain walls, due to the flexible orientational order of the organic molecules. The presence of charged domain walls will significantly reduce the band gap. We demonstrate that charged domain walls can serve as segregated channels for the diffusion of charge carriers.
Liu, Shi, "Multiscale Simulations of Dynamics of Ferroelectric Domains" (2015). Publicly Accessible Penn Dissertations. 1085.