Atomistic Simulation of Electroceramics

This work presents a range of atomistic techniques used to study the energetic and structural properties of the perovskite, barium titanate (BaTiO3). Particular attention is given to defective structures of BaTiO3 and their importance at the atomic scale and for the behaviour of the material. Using a newly developed potential model calibrated against ab initio calculations, simulations of rare earth (RE) and transition metal doping of both the cubic (space group Pm3m) and hexagonal polymorphs (space group P63/mmc) of BaTiO3 have been completed. All major dopant charge compensation schemes have been considered as well as the contribution from binding between charged defects. The results agree with simple ion size arguments and excellent agreement with experiment is observed. Clear evidence of the stabilisation of the hexagonal polymorph as a result of trivalent and tetravalent transition metal doping is presented. Lattice statics have also been used to study the energetics and structures of a range of ATiO3 solid solutions, where A is Ba, Ca and Sr. Energy of mixing curves have been produced using both the new and old potential models. The relationship between strain and the ferroelectric Curie temperature (Tc) in Ba1-xCaxTiO3 has also been considered. Molecular dynamics methods have been applied to investigate oxygen diffusion in cubic BaTiO3 and SrTiO3. Mean square displacement (MSD) calculations were completed over a range of oxygen vacancy concentrations. Diffusion coefficients and activation energies have been calculated and compared with experiment. Finally, we have performed density functional theory (DFT) simulations on mono- and di-vacancies in hexagonal BaTiO3. Defect formation energies are derived for multiple charge states and due consideration is given to the errors usually associated with such calculations. Equilibrium concentrations of vacancies in the system are also presented. Comparisons are drawn with the cubic polymorph as well as with potential-based simulations and experimental results.