Relationship between microwave dielectric properties and chemical bonding in R2BaMO5 (R = rare earth, M = Cu and Zn) compounds

The relationship between the covalency of cation–oxygen bonds and microwave dielectric properties of the green phase-type R2BaCuO5 and R2BaZnO5 (R = rare earth) compounds was investigated by using the bond valence theorem. From the calculation results of covalency, it was shown that the covalency of the RO bond in the R2BaCuO5 and R2BaZnO5 compounds decreased with increasing ionic radii of the R ions. Comparing the covalency of the RO bond in the R2BaZnO5 compound with that of the R2BaCuO5 compound, the former was smaller than the latter; therefore, this result was attributed to the differences in the RO bonds between the R2BaCuO5 and R2BaZnO5 compounds. The dielectric constant of both compounds was linearly increased from 14 to 18 with increasing the ionic radii of the R ions. Then, the dielectric constant of the R2BaZnO5 compound was large in comparison with that of the R2BaCuO5 compound, though the ionic polarizability of Zn was smaller than that of Cu. Thus, it was considered that the difference in the dielectric constant between the R2BaCuO5 and R2BaZnO5 compounds is related to the covalency of the RO bonds.