Effects of non-stoichiometry on the microstructure, oxygen vacancies, and piezoelectric properties of CuTa ₂ O ₆-doped NKN ceramics

The effects of non-stoichiometry on the microstructure, oxygen vacancies, and piezoelectric properties of (Na_0.5K_0.5) _xNbO₃ (NKxN, where x = 0.98, 1.00, 1.01, and 1.02) ceramics doped with sintering aid CuTa₂O₆ (CT) doping were investigated. X-ray diffraction (XRD) patterns indicated that a secondary phase formed in CT-doped NKxN (NKxNCT) ceramics with x < 1.00 and that a pure phase was obtained with x ≥ 1.00. The grain size of NKxNCT ceramics increased with increasing x value due to the formation of a liquid phase. The internal bias field, activation energy, and Raman analysis for NKxNCT ceramics showed that the number of induced oxygen vacancies increased with decreasing x value. The high mechanical quality factor (Q_m) value obtained for NKxNCT ceramics did not correspond to a higher concentration of oxygen vacancies, illustrating that the suitable compensation (excess Na and K) is more important than the concentration of oxygen vacancies to obtain the ceramics with high Q_m values. The NKxNCT ceramics with x = 1.01 exhibited excellent piezoelectric properties, with k_p and Q_m values of 39.9% and 2,070, respectively.