Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics

Highly dense (1−x)(Na0.5K0.5)NbO3–x(Bi0.5Na0.5)TiO3 (NKN-BST) solid solution piezoelectric ceramics have been fabricated by ordinary sintering. All compositions show pure perovskite structures, showing room-temperature symmetries of orthorhombic at x⩽0.02, of tetragonal at 0.03⩽x⩽0.09, of cubic at 0.09<x⩽0.20, and of rhombohedral at x>0.20. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was identified in the composition range of 0.02<x<0.03. The materials near the MPB exhibit a strong compositional dependence, owning peak values of the planar electromechanical coupling factor kp∼43%, the piezoelectric constant d33∼195pC∕N, and the Curie temperature of 375°C comparable to that of commercial lead zinconate titanate ceramics. The results indicate that NKN-BST ceramic is a promising lead-free piezoelectric candidate material.