Structural, dielectric, and piezoelectric properties of lead-free (1-x)K1/2Na1/2NbO3 - xCa(Zn1/3Ta2/3)O3 perovskite solid solution

Polycrystalline ceramics in (1 - x)K1/2Na1/2NbO3 - xCaZn(1/3)Ta(2/3)O(3) (abbreviated as KNN-CZT) solid solution have been fabricated by using conventional solid-state synthesis route and the effect of CZT addition on the crystal structure, dielectric, and piezoelectric behaviour of KNN has been studied. Rietveld refinement of the room temperature X-ray diffraction data confirmed the crystal structure to be a pure perovskite phase for the compositions in the range x = 0 - 0.10. Further, the crystal structure gradually changed from orthorhombic to cubic via the formation of rhombohedral and tetragonal mixed phases with the increase in x. Raman spectroscopy suggested damping of phonon modes and a strong anharmonicity in the crystal arising due to the increased disorder in the structure as a consequence of multivalent cations occupying the A and B sites in CZT substituted compositions. Dielectric behaviour indicated the shifting of both tetragonal-cubic and orthorhombic-tetragonal phase transitions towards room temperature. The temperature dependent dielectric constant was modelled by Lorentz quadratic law, and the fitted value of diffuseness parameters confirmed an increase in diffuseness of phase transition with increasing substitution. Sample with the composition corresponding to x = 0.02 showed an improved piezoelectric coefficient d(33) similar to 125 pC/N and electromechanical coupling coefficient k(p) similar to 30% at room temperature and d(33) similar to 61 pC/N and k(p) similar to 24% at 300 degrees C which make this material a potential candidate for high-temperature piezoelectric applications. Variation of voltage coefficient (g(33)) with the change in compositions are also reported. Improvement in the piezoelectric properties is attributed to the reduced oxygen vacancies.