Electrical conduction and impedance spectroscopic studies for KNbO₃–NaNbO₃ high Tc electroceramics with enhanced d₃₃ via BaTiO₃ co-doping approach

The (1 − x) K_0.5Na_0.5NbO₃–xBaTiO₃ (where x (mol.%) = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, which is abbreviated as KB1, KB2, KB3, KB4, KB5, respectively) electroceramics developed to enhance the piezoelectric properties with stabilized Curie temperature (Tc) and impedance spectroscopy analysis were carried out to investigate the conduction mechanism. The cold isostatic pressing (CIP) technique was employed to achieve a moderately dense microstructure of KNN-based ceramics. The temperature-dependent dielectric measurements show that the Curie temperature of KNN is 418 °C, whereas the KB1 ceramics is 410 °C. The Rietveld analysis carried out for all synthesized ceramics confirmed that the KB3 ceramics have orthorhombic and tetragonal (O–T) phase coexistence near room temperature. The electromechanical coupling coefficient (k_p ~ 0.22) and piezoelectric charge coefficient (d₃₃ ~ 78 pC/N) were for pure KNN ceramics, whereas for KB1 ceramics is k_p = 0.283 and d₃₃ = 126 pC/N. The temperature-dependent Nyquist plot (Z″ versus Z′) of all ceramics in the frequency range 1 Hz–15 MHz exhibited a single semicircle arc. For all ceramics, the bulk resistance (R_b) decreases with increasing the temperature from 623 to 723 K, indicating the NTCR behavior. The activation energy of KNN is 0.98 eV and in the range between 1.15 and 1.73 eV for KB1–KB5 ceramics suggesting that double-ionized oxygen vacancy is responsible for conduction mechanism. As a consequence of double-ionized oxygen vacancies, the electrical conduction in KNN and KB1–KB-5 ceramics is ionic.