Effect of isovalent substitutions on the microwave dielectric properties of Ca4La6(SiO4)4(PO4)2O2 apatite

Abstract

The isovalent substitutions of Sr2+, Pr3+, (GeO4)4− and (VO4)3− in the respective cationic and anionic sites of Ca4La6(SiO4)4(PO4)2O2 have been accomplished. The substitution of Sr2+ for Ca2+ decreases εr and Qu × f but increases the τf. The value of εr increases and Qu × f decreases when (GeO4)4− is substituted for (SiO4)4− with a marginal variation in τf. The gradual replacement of (PO4)3− with (VO4)3− brings about an increase in εr and τf with a small decrease in Qu × f. The partial substitution of Pr3+ for La3+ is found to be effective in decreasing the τf without much affecting εr and Qu × f. The composition Ca4(La4Pr2)(SiO4)4(PO4)2O2 shows good microwave dielectric properties: εr = 13.8, Qu × f = 26000 GHz and τf = −7 ppm/°C. The ceramics have low linear coefficient of thermal expansion (4–5 ppm/°C).