One Site, Two Cations, Three Environments: s² and s⁰ Electronic Configurations Generate Pb-Free Relaxor Behavior in a Perovskite Oxide

Abstract

The piezoelectric devices widespread in society use noncentrosymmetric Pb-based oxides because of their outstanding functional properties. The highest figures of merit reported are for perovskites based on the parent Pb(Mg_1/3Nb_2/3)O₃ (PMN), which is a relaxor: a centrosymmetric material with local symmetry breaking that enables functional properties, which resemble those of a noncentrosymmetric material. We present the Pb-free relaxor (K_1/2Bi_1/2)(Mg_1/3Nb_2/3)O₃ (KBMN), where the thermal and (di)electric behavior emerges from the discrete structural roles of the s⁰ K⁺ and s² Bi³⁺ cations occupying the same A site in the perovskite structure, as revealed by diffraction methods. This opens a distinctive route to Pb-free piezoelectrics based on relaxor parents, which we demonstrate in a solid solution of KBMN with the Pb-free ferroelectric (K_1/2Bi_1/2)TiO₃, where the structure and function evolve together, revealing a morphotropic phase boundary, as seen in PMN-derived systems. The detailed multiple-length-scale understanding of the functional behavior of KBMN suggests that precise chemical manipulation of the more diverse local displacements in the Pb-free relaxor will enhance performance.