Effects of structural characteristics on microwave dielectric properties of (1−x)CaWO4 –xLaNbO4 ceramics

Dependences of microwave dielectric properties on the structural characteristics of (1−x)CaWO₄–xLaNbO₄ ceramics were investigated as a function of LaNbO₄ content (0.0 ≤ x ≤ 0.5). A single phase with tetragonal scheelite structure was obtained up to x = 0.35, and then the mixture phases with scheelite and fergusonite structure were detected. With the increase of LaNbO₄, the deviation of the observed dielectric polarizabilities calculated by the Clausius-Mosotti equation from the theoretical values calculated by the additivity rule of dielectric polarizability, was decreased due to the decrease of oxygen bond valence in ABO₄ scheelite structure. Dielectric constant (K) and temperature coefficient of resonant frequency (TCF) were increased with LaNbO₄ content due to the decrease of oxygen bond valence. Q ⋅ fvalue was dependent on the atomic packing fraction of unit cell as well as the grain size. Typically, K = 13.3, Qf = 50,000 GHz and TCF = −8.7 ppm/^oC were obtained for the specimens with 0.3 mol of LaNbO₄ sintered at 1150^oC for 3 h.