Abstract: Microwave properties and crystal structures of [(Pb_0.48Ca_0.52)_1-xNd_x](Fe_0.5Nb_0.5)O₃ perovskite structure system were researched. As Nd³⁺ content
increased, the composition of specimens changed from single phase of perovskite structures to multiphase containing PbO-Nb₂O₅
pyrochlore phase whose content increased linearly with the increasing Nd³⁺ content. The dielectric constant firstly increased from 92.3
to 96.1 then gradually decreased to 68.4 as Nd³⁺ content increased. The temperature coefficient of resonant frequencies could be effectively
improved from 37.8ppm/K to 0.9ppm/K and the relationship between microwave dielectric properties and crystal structures was discussed
by using the theory of bond valence. The unloaded quality factor abruptly decreased as Nd³⁺ completely solved at A-site of the perovskite
structure then the value would decrease because of the large amount of pyrochlore phase. When x=0.05, good properties of ε_r=87.9, Qf=5210GHz and
τ_f=7.8ppm/K could be obtained in [(Pb_0.48Ca_0.52)_1-xNd_x](Fe_0.5Nb_0.5)O₃ system.

Key words: microwave properties, crystal structures, perovskite, pyrochlore phase, bond valence