Multiscale theory and simulation of barium titanate

Although barium titanate is one of the most widely studied ferroelectric materials, questions about the nature of its phase transition remain. There are two competing models for this transition (displacive and order–disorder) and experiments detect signs of both types of transition. To study this issue computationally requires the simulation of large, disordered systems on the atomic scale. In this PhD I have developed a new classical force field for BaTiO3 which, in essence, is an ionic model. The free parameters within this force field were fitted to data generated from density-functional theory (DFT) simulations. Properties that were not used in the fitting procedure calculated with the resulting force field were found to be in excellent agreement with those calculated via DFT. The potential energy surface of BaTiO3 has been explored in detail using DFT and the force field, enabling both sublattice and single ion displacements to be studied, and highlighting in particular the important role played by the oxygen ions. Dynamical simulations show behaviour compatible both with experiments that have been interpreted as evidence for displacive transitions and with experiments interpreted as evidence for order–disorder transitions. In particular, when the local structure of the phases was studied, the locations of the ions were found to be consistent with a displacive model, whereas the calculated distribution of polarisation densities was more characteristic of an order– disorder model. The direction of the local relative displacements of titanium and oxygen ions was also considered. The deviation of this displacement from the ⟨111⟩ directions produced excellent agreement with experimental measurements of the tetragonal phase, and a similar effect was found in the orthorhombic phase. These dynamical simulations were found to be very sensitive to a range of factors such as supercell size.Open Acces