Structure–property relations in xCaTiO3–(1−x)SrMg1/3Nb2/3O3 based microwave dielectrics

Solid solutions of the system (x)CaTiO3–(1−x)Sr(Mg1/3Nb2/3)O3 (CTSMN) were prepared by the mixed oxide route. All compositions were found to be single phase by X-ray diffraction; pellets had densities >95% of the theoretical X-ray density. Scanning electron microscopy coupled with energy dispersive X-ray analysis revealed a homogeneous distribution of cations within instrument sensitivity in all samples except for pure SMN which showed evidence of some unreacted MgO. SMN is reported as a 1:2 ordered trigonal/hexagonal complex perovskite structure in the X-ray data files but electron diffraction revealed superlattice reflections characteristic of both in-phase and anti-phase rotations of octahedra arising from further structural phase transitions on cooling. These reflections were retained throughout the solid solution range but ±1/3{hkl} ordered superlattice reflections were absent in samples where x > 0.1. In general, the microwave quality factor (Qu*fo) decreased as x increased and the temperature coefficient of resonant frequency (TCf) was approximately linearly proportional to the permittivity; at x∼0.20: TCf∼0, εr ∼45 and Qu*fo ∼9000.