A series of tungsten bronze-type solid solutions Ba6-3xR8+2xTi18O54 (R=La, Pr, Nd and Sm) has good microwave dielectric properties. Though the dielectric constant εr and the temperature coefficient τf of resonant frequency change linearly as a function of composition x, the quality factor Q⋅f shows nonlinear behavior of steep increase around x=0.5. The highest gradient was found in the Sm system in which the difference in ionic radius from Ba is largest. Increasing of ionic radii of R, the gradient gradually decreases. Release of inner stress which will be caused by the distribution of large ions with different sizes might bring the increase in Q⋅f. When Ba ions remain in the 2×2 perovskite blocks which is mainly occupied by R ions in the tungsten bronze structure, the inner stress might become large. Vacancies created by the substitution also bring decrease of the stress. It is also discussed that the deviation of the composition from the solid solutions decreases the sintering temperature without influence of the dielectric properties.
