An electromechanical atomic-scale finite element method for simulating evolutions of ferroelectric nanodomains

In this paper, a novel atomic-level computational method of perovskite ferroelectrics is established by combining the shell model and atomic-scale finite element method (AFEM). Its applicability is carefully testified for both bulk and nanoscale ferroelectrics, by comparing the calculated structural parameters and polarizations with the molecular dynamics (MD) simulations, first-principles calculations and experiment results. A comparison of the CPU time demonstrates that the developed method has a computational speed about 10 times over that of shell model MD method and its advantage becomes more evident as the computational scale becomes larger. Moreover, two effective calculation skills of long-range Coulomb force are introduced which can further enhance the computational efficiency by about 10 times. Using the developed atomic-level method, we investigate the various patterns of nanoscale domain structures in BaTiO3 and their evolutions under electrical loadings. A domain structure with coexistence of vortex and streamline polarization patterns is revealed. Furthermore, the simulations of domain evolutions not only reproduce well the two-step 90° domain switching process observed in experiments on single domain under an anti-parallel electric field, but also provide a full evolution diagram among different domain patterns under various electric fields. A quantitative analysis indicates that the direction-dependent coercive field of multi-domain structure can be well described by that of single domain based on a simple analytical model. This study on domain patterns and evolutions may help us understand the behaviors of ferroelectrics from the atomic level.