Fundamental workings of chemical substitution at the A-site of perovskite oxides— a ²⁰⁷Pb NMR study of Ba-substituted PbZrO₃

Lead zirconate (PbZrO₃, PZ) is a prototype antiferroelectric (AFE) oxide from which state-of-the-art energy storage materials are derived by chemical substitutions. A thorough understanding of the structure-property relationships of PZ-based materials is essential for both performance improvement and the design of more environmentally friendly replacements. (Pb_1−xBa_x)ZrO₃ (PBZ) can serve as a model system for studying the effect of A-site substitution in the perovskite lattice, with barium destabilizing the AFE state. Here, the two-dimensional ²⁰⁷Pb solid-state NMR spectra of PZ and PBZ were recorded to analyze the local structural role of barium substitution. At low substitution levels, ²⁰⁷Pb NMR spectroscopy reveals the presence of Pb-O bond length disorder. Upon crossing the threshold value of x for the macroscopic phase transition into a ferroelectric (FE) state, the barium cations cause local-scale lattice expansions in their vicinity, resulting in the collapse of two lead lattice sites into one. The stabilization of the larger volume site coincides with the favoring of larger lead displacements. We also observed more covalent bonding environments which may originate from the lower polarizability of the barium cations, facilitating the formation of stronger Pb-O bonds in their vicinity. From the local structural point of view, we propose that the substitution-induced AFE → FE phase transition is therefore related to an increasing correlation of larger lead displacements in larger oxygen cavities as the barium content increases. Our results also highlight ²⁰⁷Pb NMR spectroscopy as a valuable method for the characterization of the structure-property relationships of PbZrO₃-based AFE and FE oxides.