EFFECT OF ANTIMONY SUBSTITUTION FOR NIOBIUM ON THE CRYSTAL STRUCTURE, PIEZOELECTRIC AND DIELECTRIC PROPERTIES OF (K_0.5Na_0.5)NbO₃ CERAMICS

The effect of antimony ( Sb ) substitution for niobium ( Nb ) on potassium sodium niobate (KNN) ceramic was investigated with respect to the densification behaviour at different sintering temperatures, microstructure and electrical properties. A small amount of Sb ⁵⁺ was added while simultaneously lowering the amount of Nb ⁵⁺ and in this study of the ( K _0.5 Na _0.5)( Nb _1-x Sb _x) O ₃ system, x content was varied from 0 to 14 mol%. Our results show that Sb ⁵⁺ slightly increased the optimum sintering temperature for KNN but above 8 mol%, its resistivity and piezoelectric properties decreased. As the amount of Sb ⁵⁺ substituted is increased, the structure of the ceramic transformed from orthorhombic to pseudo-cubic which led to slight shrinkage in the unit cell volume. Microstructural examination revealed that above 10 mol%, a second phase ( K ₂ NaSb ₃ O ₉) was formed which segregated mainly to the grain boundary while the quantitative EDX analysis showed that there was A-site vacancy due to loss of the alkali elements. The two phase transitions points, Curie temperature (T _C ) and the tetragonal to orthorhombic (T _T-O ) shifted to lower temperature with increasing Sb ⁵⁺ content and above 10 mol%, the T _T-O shifted to below room temperature. The dielectric loss slightly increases with increasing Sb ⁵⁺ content up to 200°C. There was an improvement in the piezoelectric properties with ≤ 6 mol% Sb content while optimum properties were obtained with 4 mol% (K _P = 0.46, Q _m = 6.2, N _P = 2296).