Structural evolution of the R‐T phase boundary in KNN ‐based ceramics

Abstract Although a rhombohedral‐tetragonal (R‐T) phase boundary is known to substantially enhance the piezoelectric properties of potassium‐sodium niobate ceramics, the structural evolution of the R‐T phase boundary itself is still unclear. In this work, the structural evolution of R‐T phase boundary from −150°C to 200°C is investigated in (0.99− x )K 0.5 Na 0.5 Nb 1− y Sb y O 3 –0.01CaSnO 3 – x Bi 0.5 K 0.5 HfO 3 (where x = 0‐0.05 with y = 0.035, and y = 0‐0.07 with x = 0.03) ceramics. Through temperature‐dependent powder X‐ray diffraction ( XRD ) patterns and Raman spectra, the structural evolution was determined to be Rhombohedral (R, <−125°C)→Rhombohedral + Orthorhombic (R + O, −125°C to 0°C)→Rhombohedral + Tetragonal (R + T, 0 °C to 150°C)→dominating Tetragonal (T, 200°C to Curie temperature ( T C )) → Cubic (C, > T C ). In addition, the enhanced electrical properties (e.g., a direct piezoelectric coefficient ( d 33 ) of ~450 ± 5 pC/N, a conversion piezoelectric coefficient ( ) of ~580 ± 5 pm/V, an electromechanical coupling factor ( k p ) of ~0.50 ± 0.02, and T C ~250°C), fatigue‐free behavior, and good thermal stability were exhibited by the ceramics possessing the R‐T phase boundary. This work improves understanding of the physical mechanism behind the R‐T phase boundary in KNN ‐based ceramics and is an important step toward their adoption in practical applications.