Synthesis and dielectric properties of barium tantalates and niobates with complex perovskite structure

Phase composition, degree of cation ordering, and dielectric properties of complex perovskites with general formula Ba(B' _1/3B"_2/3)O₃, where B′ = Mg, Zn, and Ni and B" = Nb and Ta, were analyzed. It was found that all the studied complex perovskites attained high degrees of 1:2 cation ordering at temperatures specific to each composition. A high temperature order–disorder phase transition in Ba(Zn_1/3Nb_2/3)O₃ occurred below 1380 °C. Ba(Ni_1/3Nb_2/3)O₃ (BNN) and Ba(Mg_1/3Nb_2/3)O₃ (BMN) pervoskites remained 100% ordered at temperatures as high as 1500 and 1620 °C, respectively. It was found that in BMN and BNN extrinsic factors, such as the second phase (i.e., Ba₃Nb₅O₁₅) and point defects, dominated the dielectric loss at microwave frequencies. Ba(Mg_1/3Ta_2/3)O₃ (BMT) remained single phase up to 1630 °C. Above this temperature, the Ba₃Ta₅O₁₅ second phase was detected. A decrease in the 1:2 cation ordering and increase of dielectric loss in BMT occurred at sintering temperatures above 1590 °C. It was also revealed by electron paramagnetic resonance that all samples studied contained a substantial amount of paramagnetic point defects. These defects contributed to extrinsic dielectric loss at microwave frequencies, thus degrading the Q factor.