Hole conduction and electro-mechanical properties of Na _0.5Bi_2.5Ta₂O₉-based piezoelectric ceramics with the Li⁺/Ce³⁺/Sc³⁺ modification

Na_0.5Bi_2.5Ta₂O₉-based piezoelectric ceramics, Na_0.5-xBi_{0.5- x}Li_xCe_xBi₂Ta_{2- x}Sc_xO_9-x (NBTO-x, x = 0-0.05), were synthesized by using a solid-state reaction process, and their electro-mechanical properties and electrical conduction behaviors were investigated in detail. The Li⁺/Ce³⁺/Sc³⁺ modification improved the electro-mechanical properties of the ceramics effectively, whereas further N₂ or O₂ annealing led to no obvious increase in piezoelectric coefficient (d₃₃). The composition x = 0.03 ceramic with high temperature stability had a Curie point (T _c) of 784 °C and a d₃₃ of 25.8 pC/N, and its electromechanical coupling factors, k_p and k_t, were 11.8% and 20.7%, respectively. Variable-atmosphere (air, O₂ and N ₂) impedance data suggested that the NBTO-x ceramics were mainly p-type materials contributing from the bulk response, and the conducting species were holes (h^•). Therefore, lower bulk resistivity (ρ ) and lower activation energy (E_a) were associated with the treatment with higher P_O2 (oxygen partial pressure). In addition, the O₂ atmosphere had stronger impact on the conductivity of the pure NBTO than those of the acceptor NBTO-x, and the grain and grain boundary contributed to its resistivity together.