Effect of heat treatment on the properties of non-stoichiometric Ba3CoNb2O9 ceramics: Evaluation of crystal structure, order-disorder behavior, and dielectric characteristics

Non-stoichiometric Ba(Co1/3Nb2/3)O3 (BCN) ceramics were synthesized using the solid-state method at 1150 ℃. The heat treatment process was then followed by sintering at 1400–1550 ℃ and annealing at 1300 °C. Systematically varying the extent of the non-stoichiometry and the heat treatment conditions resulted in the division of the BaO−CoO−Nb2O5 system into two categories of compounds: (A) high-order Co/Ba-deficient and Co/Nb-excess ceramics and (B) completely disordered Nb-deficient and Ba-excess ceramics. It was found that a slight deviation from stoichiometry toward the group (A) and subsequent annealing led to a significant improvement in the ordering from 82% to 100% accompanied with the formation of non-equidistant Nb−O bonds. The crystal structure based on the ultra-high order BCN−0.02Ba5Nb4O15 was also designed to display non-equidistant Nb−O bonds. The dielectric characteristics demonstrated that high Q correlates with the 1:2 ordering and the highest Q×f values obtained from BCN−xBa5Nb4O15 ceramics (x = 0.02, 0.04, 0.06): 83, 84.95, and 79.71 THz, respectively.