Structure analysis on the Ba3Mg(Ta1−xNbx)2O9 ceramics: Coexistence of order and disorder

The Ba₃ZnTa₂O₉ (BZT) and Ba₃MgTa₂O₉ (BMT) ceramics, a family of A₃B²⁺B⁵⁺₂O₉ complex perovskites, are extensively utilized in mobile based technologies due to their intrinsic high unloaded quality factor, high dielectric constant and a low (near-zero) resonant frequency temperature coefficient at microwave frequencies. The preparation conditions as well as size and nature of B cations have a profound effect on the final dielectric properties. In this article, we report the effect of Nb⁵⁺ at the Ta⁵⁺ site on the BMT structure prepared at four synthesis temperatures (1300, 1400, 1500 and 1600 °C). The analysis has been carried out using the Rietveld technique on the X-ray powder diffraction data. Results suggest that both the preparation temperatures and Nb⁵⁺ content have significant effect on the ordering of B cations in the Ba₃Mg(Ta_1−xNb_x)₂O₉ solid solution. A disordered (cubic) structure is preferred by the 1300 °C compounds. The weight percentage of the ordered (trigonal) phase escalates, for a given composition, with increasing calcination temperature. A fully ordered trigonal arrangement exists only for x = 0.0 and 0.2 compounds calcined at 1600 °C, and the rest are biphasic (cubic and trigonal). The increase in the cubic fraction upon Nb⁵⁺ augmentation suggests that the solid solution leans more toward the disordered structural arrangement of B²⁺ and B⁵⁺ cations.