Phase Transformation Kinetics and Oxygen Transport in the Relaxor Ferroelectric Na1/2Bi1/2TiO3 studied by First-Principles Calculations

Health and environmental-friendly aspects of daily life have increased in importance over the last decades. As materials scientists, we are able to develop non-hazardous materials, which can replace the toxic materials that are currently used in piezoelectric devices. Also, the efficiencies of cooling and energy conversion devices can be improved by new materials, which are especially important in industrialized countries. In this work we tackle these aspects from a computer-aided modeling point of view. A major aim of this work is to study the inetic processes of the relaxor ferroelectric sodium bismuth titanate Na1/2Bi1/2 TiO3 (NBT) on a fundamental level by first-principles density functional theory (DFT) calculations. This material is of particular interest since its solid solutions are viable candidates to replace the toxic, however yet indispensable, lead zirconium titanate (PZT) in various piezoelectric and cooling applications. Within this thesis we investigate the influence of chemical A-cation order, octahedral tilts and cation displacements on the macroscopic properties. In particular, we study phase transformations, try to identify the origins of the relaxor ferroelectric properties, investigate oxygen migration and polar defect associates. Further, we employ Ising-like Monte Carlo (MC) simulations and ab initio based effective molecular dynamics (MD) simulation to study the electrocaloric effect (ECE) in ferroelectrics, anti-ferroelectrics and relaxor ferroelectrics. The ECE describes the increase in temperature of a material when an electric field is applied. Although the effect is rather small, it might be possible to utilize it in solid-state cooling devices in the future. Relaxor ferroelectrics are expected to increase the achievable temperature change compared to conventional ferroelectrics. It is also investigated how polar defects, which are unavoidable in devices, influence the ECE. We find that the energy landscape of NBT for octahedral tilts is very flat, thus different kinds of octahedral tilt patterns can easily be introduced and the underlying order of the A-cations is very important. These local deviations in tilt pattern then can lead to locally varying A-cation displacements which result in different polarizations, leading to the observable relaxor properties. The phase and symmetry transitions also influence the ionic conductivity and defect associates. Further, our employed MC and MD calculations could not show any increase of the ECE for relaxors, compared to conventional ferroelectrics.However, the simple Ising-like model proved to be sufficient to explain the negative electrocaloric temperature change observed in anti-ferroelectrics and ferroelectrics containing polar defects