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 degrees C to 200 degrees C is investigated in (0.99-x)K0.5Na0.5Nb1-ySbyO3-0.01CaSnO(3)-xBi(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 degrees C)Rhombohedral + Orthorhombic (R + O, -125 degrees C to 0 degrees C)Rhombohedral + Tetragonal (R + T, 0 degrees C to 150 degrees C)dominating Tetragonal (T, 200 degrees 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 (d33) of -580 +/- 5 pm/V, an electromechanical coupling factor (k(p)) of -0.50 +/- 0.02, and T-C-250 degrees 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.