Phase Relations and Dielectric Properties in the Bi₂O₃-ZnO-Ta₂O₅ System

Dielectric properties and phase formation of Bi-based pyrochlore ceramics were evaluated for the Bi₂O₃-ZnO-Ta₂O₅ system. The compositional range rBi₂(Zn,_1/3Ta_2/3,)₂O ₇· (1-r)(Bi_3/2Zn_1/2)(Zn_1/2Ta_3/2)O ₇ (0 ≤ r ≤ 1) in Bi₂O3_- ZnO-Ta₂O₅ was investigated to determine the relative solubility of BZT cubic (α-BZT, r = 0) and the pseudo-orthorhombic (β-BZT, r = 1) end members. It was found that extrinsic factors, such as kinetically limited phase formation and bismuth loss, contribute to apparent phase boundaries in addition to thermodynamic stability of each phase. Considering this, the locations of true phase boundaries were r < 0.30 and r ≥ 0.74 for α and β phases, respectively. Dielectric constants between 58 and 80 and low dielectric loss (tan δ < 0.003) were measured for the complete compositional range. The temperature coefficient of capacitance was controlled by composition, which was found to be <30 ppm/°C at the edge of β-phase solid solution. In addition to the excellent dielectric properties these materials can be sintered at low temperatures, which make Bi-based pyrochlores promising candidates for high-frequency electronic applications.