Synergistically Enhanced Electrical Properties in BCST Lead-free Piezoelectric Ceramics via Defect-driven Lattice Distortion within the Polymorphic Phase Boundary

To meet the growing demand for environmentally friendly electronic components such as sensors and actuators, the development of lead-free ceramics with excellent electrical properties is of great importance. In this work, heterovalent Al 3+ ions were introduced into triphase-coexisting (Ba 0.94 Ca 0.06 )(Ti 0.94 Sn 0.06 )O 3 -based ceramics via a conventional solid-state reaction method, successfully constructing (1−x)(Ba 0.94 Ca 0.06 )(Ti 0.94 Sn 0.06 )O 3–x Al 2 O 3 lead-free piezoelectric ceramics featuring the coexistence of defect structures and multiple phases, including rhombohedral (R), orthorhombic (O), and tetragonal (T) phases. The experimental results demonstrate that the ceramic with an Al 2 O 3 doping concentration of 0.3 mol% exhibits optimal comprehensive electrical performance, with d 33 = 583 ± 20 pC/N, k p = 43%, ε r = 3296, and tanδ = 1.773%. Structural and chemical characterizations based on X-ray diffraction (XRD), temperature-dependent dielectric measurements, and X-ray photoelectron spectroscopy (XPS) reveal that the outstanding electrical performance originates from pronounced lattice distortion and a stabilized triphase coexistence governed by an extremely low Gibbs free-energy difference. This study demonstrates that precise defect regulation within a triphase-coexisting matrix is an effective strategy to induce favorable lattice distortion and synergistically enhance the electrical properties of lead-free piezoelectric ceramics.