Piezoelectric properties, phase transitions and domain structure of (Bi,Na)HfO3-modified (K,Na)(Nb,Sb)O3 ceramics

Piezoelectric performances of (K,Na)NbO3-based ceramics are often greatly affected by phase transitions and domain structure, which are in turn substantially determined by chemical compositions. Systematical studies from this perspective are important for the good understanding about piezoelectric characters. Further, whether rhombohedral-tetragonal (R-T) or rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence is really the essential precondition for obtaining high piezoelectric coefficient d(33) is also a fundamental issue. A series of (1-x)(K0.48Na0.52)Nb0.955Sb0.045O3-x(Bi0.50Na0.50)HfO3 ceramics with 0.01 <= x <= 0.04 were chosen in this work to examine the compositional effects on piezoelectric properties, phase transitions and domain structure. It was found that piezoelectric coefficient d(33) , orthorhombic-tetragonal (O-T) phase transition and domain structure are sensitive to the chemical composition. The d(33) increases first, reaches a maximum of 512 pC/N at x = 0.037 and then decreases with x . Dielectric measurement showed that the O-T phase transition temperature TO-T decreases monotonically with x while the rhombohedral-orthorhombic phase transition temperature TR-O remains almost unchanged when x >= 0.03. Domain structure was investigated by acid etching. All the compositional ceramics exhibit quite complex domain patterns consisting of long parallel stripes and irregularly-shaped domain boundaries before poling. Domain patterns become less complicated upon poling due to the collective polarization reversals of domain clusters. The ceramic with x = 0.037 differs significantly from the others, showing its TO-T close to room temperature and possessing the unique domain structure that contains a large amount of hierarchical nanodomain structure. Its outstanding piezoelectric properties should be ascribed to the O-T phase coexistence and the characteristic domain structure.