Microwave Dielectric Properties of Ca4(La4Pr2)(SiO4)4(PO4)2O2 Ceramics Doped with Isovalent and Aliovalent Ions

Isovalent and aliovalent substitutions in the cationic and anionic sites of Ca4(La4Pr2)(SiO4)4(PO4)2O2 and Ba4(La4Pr2)(SiO4)4(PO4)2O2 ceramics were explored in this study. Substitutions of (SiO4)4− ions with (GeO4)4− in Ca4(La4Pr2)(SiO4)4(PO4)2O2 ceramic increased its densification temperature and thus led to a significant grain growth; however, replacement of Ca2+ and (PO4)3− ions by Mg2+ ions and (VO4)3− ions, respectively, decreased its densification temperature and resulted in grain refinement. Various rare-earth-metal and (WO4)2− ion substitutions on La3+ and Pr3+ and (PO4)3− sites, respectively, of Ca4(La4Pr2)(SiO4)4(PO4)2O2 ceramic had no impact on the sintering temperature. In the case of Ca4(La4Pr2)(SiO4)4(PO4)2O2 apatite with Mg2+ and (WO4)2− ion substitutions, second phases were observed in the x-ray diffraction patterns; however, only the pure hexagonal apatite phase with space group P63/m was visible in all the other systems. Overall, Ca4(La2Nd2Pr2)(SiO4)4(PO4)2O2 ceramic has the best microwave dielectric properties including εr = 14.2, Q × f = 28,745 GHz and τf = 0.9 ppm/°C. The densification temperatures of Ba4(La4Pr2)(SiO4)4(PO4)2O2 ceramics decreased as the SiO44− and PO43− ions were substituted by WO42− ions. The formation of the secondary phases BaWO4 and Ba2SiO4 decreased their dielectric constant and increased their Q × f values.