Crystallization and conductivity of 2CaO-La2O3-5P2O5 glass-ceramic with Al2O3 addition

Al2O3 was added to a 2CaO-La2O3-5P2O5 metaphosphate, to replace 10% of the Ca2+ ions by Al3+, forming a phosphate with the nominal composition 1.8CaO-0.1Al2O3-La2O3-5P2O5. The effect of Al2O3 addition and heat treatment on the microstructure and conductivity of the resulting glass-ceramics was investigated by XRD, SEM, TEM, and AC impedance spectroscopy. Upon transformation from glass to glass-ceramic, conductivities increased significantly. The glasses were isochronally transformed at 700 and at 800 °C for 1 h or 5 h, in air, following heating at 3 or 10 °C/min. With Al2O3 addition, after a heat treatment at 700 °C, 100-300 nm nano-domains of LaP3O9 crystallized from the glass matrix. Annealing at 800 °C produced a further order of magnitude conductivity increase for the Al-free glass, but less so for the Al-containing glass.