Diffuse ferroelectric phase transitions in Pb-substituted PbFe_1/2∕Nb_1/2O₃

Abstract

We investigate effects of cation (K, Ca, Sr, Ba, La, Bi, Tl, Ag) substitution at Pb-site in PbFe_1/2∕Nb_1/2O₃ (PFN) on its diffuse ferroelectric phase transition (DPT) through measurements and modeling of the temperature dependence of its dielectric constant. We find that the chemical trends in experimentally determined rates of change of transition temperature and diffuseness with substituent concentration are governed by two parameters, nominal and Born effective charges calculated within first-principles density functional theory. We introduce “ferroactivity” of an atom using these charges and use it in a generalized Weiss molecular field theory that accounts for the disorder to explain chemical trends in the DPT found experimentally, suggesting guidelines to tune properties of ferroelectrics.