Effect of Ta content on phase structure and electrical properties of piezoelectric lead-free [(Na_0.535K_0.480) _0.942Li_0.058](Nb_1-xTa_x)O ₃ ceramics

The Li/Ta-codoped lead-free (Na,K)NbO₃ ceramics with Na:K ratio of 0.535:0.480 were prepared by normal sintering, whose nominal formula was noted as [(Na_0.535K_0.480)_0.942Li _0.058](Nb_1-xTa_x)O₃ (x=2, 4, 6, 8, 10, 12, 14, and 16 at.%). The X-ray diffraction patterns and the corresponding calculation of lattice parameters showed that a morphotropic phase boundary crossing orthorhombic and tetragonal symmetries occurs at the compositions containing 6∼8 at.% Ta. Because of such transitional behavior achieved by optimizing Ta content, the piezoelectric coefficient (d₃₃), electromechanical coupling coefficient (k_p), and dielectric constant (ε) were enhanced to peak values, 232 pC/N, 39.4%, and 710, respectively. However, the Curie temperature (T_C) and remanent polarization (P _r) show a decreasing tendency with raising Ta content, and their maximum values are 462°C and 8.73 μC/cm², respectively.