Research master thesis: Why is calcium titanate antiferrodistortive, while lead zirconate is antiferroelectric?

Abstract

Antiferroelectric (AFE) materials have been of interest in the application of high-energy-density capacitors and strain-induced actuators. However, there are only a few identified AFE materials, most belong to oxide perovskites, such as PbZrO3, PbHfO3, and NaNbO3. We rationalize the limited number by providing their restricted criteria. The ground state of an AFE is nonpolar and experiences a phase transition to a symmetry-related polar phase under a sufficiently large electric field. To exhibit the phase transition, the energy of the polar phase should be close to the nonpolar ground state and their energy barrier should be flat enough. For the nonpolar perovskites, it is commonplace that the high energy barrier hinders the AFE characteristic. The purpose of the thesis is to understand the microscopic origin of the flat energy barrier in the prototypical AFE material, PbZrO3. We make this by comparing PbZrO3 to CaTiO3 in the second-principles models since these two materials share similar structures at high temperatures. We identify that the difference in atomic interactions, Pb-O in PbZrO3 and Ca-O in CaTiO3, is crucial for the energy barrier between the nonpolar ground states and the polar phases. Additionally, by tuning the coefficients, we reproduce the nonpolar-polar phase transition path of PbZrO3 on the CaTiO3 second-principles model. The Pb-O interactions cannot be reproduced by the hydrostatic pressure in CaTiO3.