Precursor glass stability, microstructure and ionic conductivity of glass-ceramics from the Na1+xAlxGe2-x(PO4)3 NASICON series

Glass-ceramics from the Na1+xAlxGe2-x(PO4)(3) system were synthesized for 0 <= x <= 1.0. The stability parameters of precursor glasses were analyzed indicating that systematic additions of aluminum in the NaGe2(PO4)(3) composition improve the precursor glass forming ability. Considering the glass-ceramics, it is observed that an increase in the aluminum content, x, causes a decrease in the activation energy for conduction, and thus an increase in the ionic conductivity. As x increases, beyond the corresponding increase in the charge carrier (Na+) concentration, it is also observed an increase in the unit cell volume of the NASICON structure, which may also contribute to the enhancement of ionic conductivity. Concerning the microstructure, the addition of aluminum provokes an increase in the average grain-size of the obtained glass-ceramics. Ionic conductivity increases up to x = 0.8 but remains constant within the experimental error for the x = 1.0 sample, probably due to a limit in the solid solution range. Ionic conductivity of composition x = 0.8 is still improved by applying higher crystallization temperatures, which decreases the activation energy for ionic conduction.