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