A-site cation controlled localization of dipole correlations in a relaxor material

In this work, we carry out molecular dynamics simulations based on an extended version of the bond valence force field to investigate the underlying physics of relaxor dielectrics with static temperature-stable dielectric response. We focus on the perovskite solid solution $(1{-}x){Ba}0.95{Ca}0.05{TiO}₃{-}(x){Bi}0.5{MeO}₃(Me={Mg}1/2{Ti}1/2,Sc,{Zn}1/2{Ti}1/2)$, which has a flat dielectric response over a wide temperature range which was recently observed experimentally. Our molecular dynamics simulations can not only reproduce the temperature-dielectric relationship observed in experiments, but also provide an atomic scale explanation. Specifically, doping Bi(Mg1/2Ti1/2)O₃ into the (BaCa)TiO₃ crystal reduces the spatial correlation length of electric dipole orientations to the minimum distance, which is only within the neighboring unit cells. This nearest-neighbor correlation even persists at high temperatures. As a result, in this perovskite solid solution, the correlation length and macroscopic polarization change little with the temperature, leading to temperature-stable relaxor dielectric responses. The relationships between our theory and previously proposed models, such as the independent local mode and correlated rattling cations model, are also discussed. This theoretical work aims to deepen our understanding of the theory of connecting between atomic composition and temperature-stable relaxor dielectric response.