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-1/3'B-2/3")O-3, 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(Zn1/3Nb2/3)O-3 occurred below 1380 degreesC. Ba(Ni1/3Nb2/3)O-3 (BNN) and Ba(Mg1/3Nb2/3)O-3 (BMN) pervoskites remained 100% ordered at temperatures as high as 1500 and 1620 degreesC, 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(Mg1/3Ta2/3)O-3 (BMT) remained single phase up to 1630 degreesC. 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 degreesC. It was also revealed by electron paramagnetic resonance that all samples studied contained a substantial amount of paramagnetic point defects. These defect contributed to extrinsic dielectric loss at microwave frequencies, thus degrading the Q factor.