IMF2017

By means of first-principles calculations different chemical compositions of (Ba,Ca)(Ti,Zr)O₃ solid solutions are investigated by focusing on the properties arising from the lattice dynamics. Our goal is to better understand the microscopic mechanisms involved in the appearence and competition of different phases and to rationalise the search of optimal compositions for large electrochemical response. We start by analyzing the four parents bulky compounds in order to clarify the dynamical properties and energetics at play between various phases. Then, a systematic characterization of the binary sistems BaTiO₃-CaTiO₃ and BaTiO₃-BaZrO₃ is carried out within both the Virtual Crystal Approximation (VCA) and standard Density Functional Theory (DFT) based on supercell calculations. It results that VCA is not appropriate to properly describe effects coming from both (Ba²⁺-Ca²⁺) and (Ti⁴⁺-Zr⁴⁺) substitutions. The failing of this approach is especially evident for Ba(Ti,Zr)O₃ system, where local correlations due to specific atomic and geometrical ordering give rise to unexpected polar activation of zirconium. A simple description of these mechanisms is achieved via a basic electrostatic model based on BaZrO₃/mBaTiO₃ supercells.