Short-Range Structural Correlations and Property Anomalies in Non-MPB Compositions of the Pb-Free Piezoelectric Na_0.5Bi_0.5TiO₃-K_0.5Bi_0.5TiO₃

Among Pb-free piezoelectrics, Na_0.5Bi_0.5TiO₃ (NBT)-based ferroelectric systems have a unique distinction of offering a complex interplay of ferroelectric and nonferroelectric (in-phase octahedral tilt) structural instabilities, influencing its ferroelectric, dielectric, and electromechanical behavior. The morphotropic phase boundary (MPB) system like (1 - <i>x</i>)Na_0.5Bi_0.5TiO₃-<i>x</i>K_0.5Bi_0.5TiO₃ (NBT-KBT) is where this complexity plays out in full measure. While consistent with the conventional view of the MPB with a consensus regarding the existence of composition driven rhombohedral-tetragonal ferroelectric-ferroelectric instability at <i>x</i> ≈ 0.20, this system presents a puzzle by exhibiting considerable structural disorder and property anomalies at <i>x</i> ≈ 0.40 within the tetragonal ferroelectric composition regime. Here, we exploit the mutual complementarity of X-ray diffraction, neutron powder diffraction, and electron diffraction and microscopy techniques to gain comprehensive structural insights at the global and local length scales. We found that the anomalous behavior is associated with a simultaneous onset of (i) a short-ranged rhombohedral ferroelectric correlations and (ii) a relatively long-ranged in-phase tilt distortion. The consequent spatial fluctuation in the local polar displacements, as imaged with the high-angle annular dark-field technique, provides the necessary structural insights regarding the unusual property anomalies.