Ba_6-3XNd_8+2XTi₁₈O₅₄ microwave dielectric resonators

The Ba_6-3xNd_8+2xTi₁₈O₅₄ solid solution (BNT) is well established as a useful microwave resonator material due to its high dielectric constant (ε_r), high quality factor (Q = 1/tanσ), and tunable temperature coefficient of resonant frequency (τ_f). Its crystal structure is similar to that of the tungsten bronzes, containing large pentagonal channels parallel to [001], which are only partly occupied by the divalent species. These pentagonal sites are separated by Nd⁺³ and TiO₆ octahedra arranged in an essentially perovskite-like manner. Ceramics of this phase have been processed in this study by a mixed-oxide route, and the variation in properties caused by doping with either Ca, Sr, or Pb was investigated. Most samples showed a multiphase character, with Nd₄Ti₉C₂₄ (ss) being the main secondary phase. For x < 0.75, single-phase BNT was always obtained, while phase purity was only attained at 1400°C when x = 0.75. Doping with PbO was found to usefully reduce τ_f whilst leaving both ε_r and Q at reasonable levels. Pb-doping lead to a moderately lower Q and, contrary to earlier reports, slightly lowered ε_r.