New Na₁₊Ge₂(SiO₄)(PO₄)_3- NASICON Series with Improved Grain and Grain Boundary Conductivities

In this study, we synthesize glass-ceramics of the new Na₁₊Ge₂(SiO₄)(PO₄)_3- NASICON (Na super-ionic conductor) series to evaluate the effect of Si⁴⁺/P⁵⁺ substitution on the structural, microstructural, and electrical properties of the NaGe₂(PO₄)₃ system. From X-ray diffraction, the presence of the NASICON phase is confirmed in all glass-ceramics. An expansion of the unit cell volume suggesting an increase in the bottleneck of the NASICON structure is also observed. Impedance spectroscopy allowed the separation of grain and grain boundary contributions. We observe that the grain conductivity is higher than the specific grain boundary conductivity in all of the investigated compositions (0 ≤ x ≤ 0.8). The Si⁴⁺/P⁵⁺ substitution causes an enhancement of about 2 and 3 orders of magnitude in the grain and specific grain boundary conductivities, respectively. This behavior is attributable to the introduction of new charge carriers (Na⁺) in the NASICON structure and a decrease in the activation energy. Finally, the lowest activation energy for grain (0.586 eV) is observed in the x = 0.6 sample, which indicates the easiest displacement of ions in the investigated series, suggesting that this composition presents the most suitable bottleneck size for (Na⁺) sodium ion conduction.