Interaction of point defects in ferroelectrics -parameter identification and simulation

Ferroelectric materials are used in a wide field of applications, where they are exposed to a high number of mechanical and electrical load cycles. This involves degradation of the material and a decrease of the electromechanical coupling capability, which is usually called electric fatigue. The causes are assumed to be ionic and electronic charge carriers, which interact with each other, with microstructural elements in the bulk and with interfaces. Accumulation of defects can lead to degradation, mechanical damage and dissociation reactions, for more details see e.g. [3]. In order to get a better understanding of the defect accumulation processes, a model based on material forces is used in [6] to simulate the interaction of defects in periodic and in infinite cells. Applying thermodynamically reasonable kinetic laws, defect migration is simulated in a deterministic way in order to understand the general tendency of defect formations. The transversally isotropic material is modelled with linear electromechanical coupling. Here, the defect parameters used in the continuum model are obtained by fitting the results of molecular dynamics (MD) simulations to the continuous spatial fields. Transferring data from the atomic to the continuum level is a field of active research and no unique solution can be presented. On the atomic level, Coulomb–interaction causes a displacement field incompatible to an elastic solution. To address this difficulty, the volume change of a domain around the defect is used to determine defect parameters. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)