Novel LiCa(PO3)3 and LiSr(PO3)3 microwave dielectric ceramics

Novel LiCa(PO3)3 and LiSr(PO3)3 phases were produced by conventional solid-state synthesis and formed into ceramic pellets followed by conventional sintering. The crystal structure, ceramic microstructure, microwave dielectric properties and ac conductivity were investigated. A triclinic space group P1- was obtained from Rietveld refinements, and the lattice vibration modes were determined by Raman spectroscopy. The optimum sintering conditions for LiCa(PO3)3 and LiSr(PO3)3 ceramics were 700 degrees C and 625 degrees C, respectively, both for 10 h. Dense glass-free ceramic microstructures were confirmed by SEM. High relative densities of 94.6 % and 93.8 %, low microwave epsilon r values of 5.85 and 5.62 at 14 GHz, and high quality factors Qxf of 25,200 and 23,450 GHz at 14 GHz were detected. Large negative tau f of -105 and -106 ppm/degrees C were obtained, which are uncommon but highly valuable for offsetting positive tau f values of other systems. Moderate Li+-ion conduction was detected in both compounds by ac impedance spectroscopy.