First-Principles Based Studies of Complex Oxide Materials

Since their first discovery over 70 years ago, complex ferroelectric oxides have been shown to exhibit a wide variety of physical and chemical properties that enable their use in a broad range of applications. In particular, the dynamics in a class of complex ferroelectric solid solutions known as relaxors have been under investigation due to their glass-like behavior and excellent technological properties. Nevertheless, despite over 50 years of research, the key bonding motifs of relaxor ferroelectrics that give rise to their fascinating properties such as ultrahigh piezoelectric coefficients, high permittivity over a broad temperature range, diffuse phase transition without macroscopic lattice symmetry, strong frequency dependence in dielectric response, and phonon anomalies are still subject to debate. We use molecular dynamics simulations for the prototypical Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ relaxor material to examine the structure, bonding and the spatial and temporal correlations of relaxors. Our simulations show that the unusual properties of relaxors simply stem from the presence of a multidomain state with an extremely small, ≈2-10 nm domain size and that dynamics of relaxors exhibit striking similarities to those of water, arising from the local-environment dependent dipole decoupling due to chemical bonding variation at particular atomic sites.

Ferroelectric materials have also recently attracted increased attention as a candidate class of materials for use in photovoltaic devices. In a series of computational studies of a variety of ferroelectric perovskite oxides, we showed that the physical behaviors of these materials are governed by the simple local structure and chemical bonding characteristics; these can then be modified to rationally design new materials with light absorption through the visible range that enables the use of these materials in photovoltaic applications.