Study of polymorphic phase boundary in (Na,K,Li) (Nb,Ta,Sb)O₃ piezoelectric ceramics

Li, Ta and Sb co-doped (Na,K)NbO₃ ceramics have been widely investigated due to the significantly enhanced piezoelectric properties attributed to the polymorphic phase transition (PPT). An empirical discriminant k = x/0.3 + y/0.3 + z/0.06 to estimate the PPT composition of Li_z(Na_0.5+δK_0.5−δ)_1−zNb_1−x−yTa_xSb_yO₃ ceramics was deduced based on a displayed phase diagram between the phase structure and doping contents. The phase structure could be conveniently estimated by k, in which a PPT appeared in the vicinity of k = 1, crossing O symmetry at k < 1 and T symmetry at k > 1. Around k = 1, a series of Li_zNa_0.52K_0.48−zNb_0.90Ta_0.05Sb_0.05O₃ ceramics was prepared by a traditional solid-state reaction in a narrow range 0.03 ⩽ z ⩽ 0.05. A PPT at room temperature appeared critically in the z = 0.04 composition, indicating a good feasibility of the suggested empirical equation to determine the critical PPT composition in the present study. The optimized piezoelectric properties d₃₃ 255 pC N⁻¹ and k_p 35.0% were attained due to the PPT behaviour. (paper)