Computer modelling of barium titanate

Barium titanate is an important material for both scientific and industrial applications. Much of the interest centres around its ferroelectric properties, since it is structurally the most simple in this class. In addition rare-earth doped ceramics are found to exhibit a very steep positive temperature coefficient of resistance (PTCR) above the Curie point. The theory of this PTCR effect is to date imperfectly understood. Originally a band model was proposed which has recently been interpreted in terms of a point defect structure model involving redistribution of barium vacancies in the grain boundary region. We have undertaken a comprehensive survey using theoretical techniques of the defect structure of BaTiO3. Our model suggests that titanium vacancies are more likely to be involved in the PTCR effect. Until recently this was incompatible with the experimental evidence, but a detailed investigation by Jonker and Havinga has established that titanium vacancies are the predominant compensating cation vacancy. Our results are presented in the light of the experimental evidence and the implications for the model of the PTCR effect are discussed.