Effects of ion occupancy and polarizability on the crystal structure and microwave dielectric properties of CaEuCO4 (CGa, Al) ceramics

CaEu C O 4 ( C Ga, Al) microwave dielectric ceramics were produced through a solid-state reaction. Orthorhombic olivine CaEuGaO 4 ( pnma ) exhibits a lower permittivity ( ε r  = 9.7), a higher quality factor ( Q  ×  f  = 82,049 GHz), and a larger negative resonant frequency temperature coefficient ( τ f  = −54.6 ppm/°C). CaEuAlO 4 , which has a similar chemical formula, exhibits a perovskite-like structure ( I 4/ mmm , K 2 NiF 4 -type), and its microwave dielectric properties are ε r  = 18.9, Q  ×  f  = 43,132 GHz, and τ f  = −1.6 ppm/°C. The difference in their structures is mainly caused by the properties of the Ga (III) atom. The difference in their dielectric properties can be proved to be affected by the “rattling” effect of Eu 3+ through the bond valence theory. The relationship between the covalence ( f c ) and the bond strength ( S ) was investigated, and the higher bond strength might be the main reason for the higher Q  ×  f in CaEuGaO 4 ceramic.