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

The isovalent substitutions of Sr2+, Pr3+, (GeO4)(4) and (VO4)(3) in the respective cationic and anionic sites of Ca4La6(SiO4)(4)(PO4)(2)O-2 have been accomplished. The substitution of Sr2+ for Ca2+ decreases epsilon(r) and Q(u) x f but increases the tau(f). The value of epsilon(r) increases and Q(u) x f decreases when (GeO4)(4) is substituted for (SiO4)(4) with a marginal variation in tau(f). The gradual replacement of (PO4)(3) with (VO4)(3) brings about an increase in epsilon(r) and tau(f) with a small decrease in Q(u) x f. The partial substitution of Pr3+ for La3+ is found to be effective in decreasing the tau(f) without much affecting epsilon(r) and Q(u) x f. The composition Ca-4(La4Pr2) (SiO4)(4)(PO4)(2)O-2 shows good microwave dielectric properties: epsilon(r) = 13.8, Q(u) x f = 26000 GHz and tau(f) = -7 ppm/degrees C. The ceramics have low linear coefficient of thermal expansion (4-5 ppm/degrees C). (C) 2012 Elsevier B.V. All rights reserved.