Ultralow Dielectric Loss with Excellent Thermal Stability in BiScO ₃ –PbTiO ₃ Piezoelectric Ceramics via Phase Structure Regulation

BiScO₃-PbTiO₃ (BS-PT) ceramics combine high piezoelectricity and a high Curie temperature but are limited in power applications by high dielectric loss (tan δ ≈ 4%) and low mechanical quality factor (<i>Q</i>_m ≈ 28). Here, we introduce relaxor ferroelectric Bi(Mn_2/3Nb_1/3)O₃ (BMN) to construct a T-rich morphotropic phase boundary (MPB), achieving simultaneous reduction of tan δ and enhancement of <i>Q</i>_m without significant degradation of piezoelectric properties. The optimized 0.02BMN-BS-0.635PT composition delivers tan δ = 0.57%, <i>d</i>₃₃ = 365 pC/N, <i>Q</i>_m = 124, <i>k</i>_p = 0.53, <i>k</i>_t = 0.54, and <i>T</i>_c = 435 °C. Mn-substitution at the B-site generates oxygen vacancies that couple with receptor centers to form defective dipoles, and their strong interaction with the T-rich phase suppresses polarization rotation and domain-wall motion, yielding reduced dielectric loss and enhanced <i>Q</i>_m, while relaxor-induced polar nanodomains help maintain a high <i>d</i>₃₃ value. In situ electrical and structural characterizations demonstrate excellent thermal stability: <i>k</i>_p and <i>k</i>_t remain 0.5 at 300 °C with <7% variation, and <i>d</i>₃₃ increases continuously up to 300 °C, indicating a depolarization temperature above this threshold. High-temperature XRD further shows that the rhombohedral-to-tetragonal phase ratio is stable below 300 °C, consistent with the T-rich MPB design at room temperature, accounting for the observed robustness. This study demonstrates the potential of BMN-BS-PT ceramics for high-temperature, high-power piezoelectric applications and establishes a generalizable phase structure regulation strategy for designing advanced piezoelectric ceramics.