Structure and microwave dielectric characteristics of Sr2[Ti1-x(Al0.5Nb0.5)x]O4 (x ≤ 0.50) ceramics

Sr-2[Ti1-x(Al0.5Nb0.5)(x)]O-4 (x = 0, 0.10, 0.25, 0.30, 0.5) ceramics were synthesized by a standard solid-state reaction process. Sr-2[Ti1-x(Al0.5Nb0.5)(x)]O-4 solid solutions with tetragonal Ruddlesdon-Popper (R-P) structure in space group I4/mmm were obtained within x <= 0.50, and only minor amount (1-2 wt%) of Sr3Ti2O7 secondary phase was detected for the compositions x >= 0.25. The temperature coefficient of resonant frequency tau(f) of Sr-2[Ti1-x(Al0.5Nb0.5)(x)]O-4 ceramics was significantly improved from 132 to 14 ppm/degrees C correlated with the increase in degree of covalency (%) with increasing x. The dielectric constant e(r) decreased linearly with increasing x, while high Qf value was maintained though it decreased firstly. The variation tendency of Qf value was dependent on the trend of packing fraction combined with the microstructure. Good combination of microwave dielectric properties was achieved for x = 0.50: e(r) = 25.1, Qf = 77 580 GHz, tau(f) = 14 ppm/degrees C. The present ceramics could be expected as new candidates of ultra-high Q microwave dielectric materials without noble element such as Ta.