Complex lead-based perovskites with the general formula Pb(B_x'B_1-x")O₃, represent an important class of antiferroelectrics beyond the prototypical PbZrO₃ and NaNbO₃. Depending on the combination of B-site species and the degree of cationic ordering, these materials exhibit a wide range of ferroic behaviors, spanning from antiferroelectric to (relaxor) ferroelectric responses. In this study, we investigate (Pb_1-xBa_x)(Yb_1/2Nb_1/2)O₃ polycrystals synthesized via a two-step processing route. Despite displaying antiferroelectric, ferroelectric, or nearly linear dielectric behavior at room temperature, all compositions exhibit double polarization hysteresis loops in proximity to a lower-temperature dielectric anomaly. This dielectric anomaly originates from the competition between antipolar and nanoscale polar regions, and shifts towards lower temperature with increasing Ba content, reflecting the suppression of long-range antiferroelectric ordering. Notably, a composition-invariant temperature scale, T*, is identified and associated with the onset of static correlations among nanoscale polar entities, consistent with behavior reported in other complex Pb-based relaxor ferroelectrics. Superlattice reflections arising from antiparallel Pb²⁺ displacements persist above T*, suggesting an intricate cation-ordering landscape requiring further investigation. These findings underscore the coexistence and competition of polar and antipolar instabilities in complex lead-based perovskites, and their pronounced sensitivity to chemical substitution, thermal fluctuations, and external electric fields.

Understanding the atomic-scale structural dynamics that enable ultrahigh piezoelectric responses in lead-free piezoceramics remains a central challenge in materials science. Here, we employ in situ electric field pair distribution function (PDF) analysis to elucidate the local structural origin of a KNN-based piezoceramic with a nominal composition of 0.964K_0.5Na_0.5Nb_0.965Sb_0.035O₃–0.03(Bi_0.5Na_0.5)_0.9(Ga_0.5Li_0.5)_0.1ZrO₃–0.006BiFeO₃ that has an exceptional piezoelectricity coefficient (d₃₃ > 500 pC/N). Combined Rietveld refinement, PDF fitting, and reverse Monte Carlo simulations revealed the coexistence of long-range tetragonal and orthorhombic phases with local c-type monoclinic symmetry. In situ electric field PDF analyses indicated a reversible polarization rotation between the <001>_PC and <110>_PC directions via a monoclinic plane, with a critical switching field of approximately 0.4 kV/mm. These findings establish polarization rotation, rather than abrupt phase transitions, as the governing mechanism for the enhanced piezoresponse, providing a structural design principle for next-generation lead-free piezoelectrics.