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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.

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Perovskite Microwave Dielectric Ceralnics: Structure, Properties and Processing PhD thesis, Phillip M. Mallinson, University of Liverpool This thesis describes the synthesis and characterisation of several new hexagonal perovskite materials and the investigation into the processing of a commercial microwave dielectric. Chapter 1 is in two parts, the first gives an introduction to the perovskite and hexagonal perovskite structural types, and reviews the structure and properties of reported hexagonal perovskites with the AnBn-\03n general formula. The second part reviews the literature on the microwave dielectric material Ba3ZnTa209 (BZT). In Chapter 2 the details of the synthetic and analytical techniques employed are described. Chapter 3 describes the synthesis and characterisation of a number of new hexagonal perovskite materials. The eight layer compounds BagCoNb60 24 and BagCoTa6024 were found to crystallise with different structures in the space groups P 3ml and P63c11l respectively. A detailed structural description and comparison of the structures of the materials is given along with the dielectric and magnetic properties. The structure and dielectric properties of the six layer material Ba6Ca1l3Nb\4/30\g are also presented. Chapter 4 describes the synthesis and characterisation of two isostructural ten layer hexagonal perovskites with the formulas BalOMgo.25Ta7.903o and BaIOCoO.25Ta7.903o. Refinement of combined synchrotron X-ray and neutron powder diffraction data is used to determine the structures of the materials. The dielectric properties of the materials are reported and the links between dielectric loss and ordering and microstructure discussed. In Chapter 5 the results of an in-situ X-ray powder diffraction study of the ordering and domain growth ofBZT at temperatures between 1200 and 1500 °C is presented. The degree of ordering is quantified using two different methods and a rate of ordering calculated at each of the temperatures studied, from the rates of ordering the activation energy for cation transport is calculated. The ordered domain size is also quantified from the diffraction data and the dynamics compared to domain growth in other systems. Supplied by The British Library - 'The world's knowledge'

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