Sol-gel synthesis of NASICON: 1D and 2D NMR investigation

The preparation of NASICON powders has been achieved by sol-gel methods. The precursor solution was either rapidly hydrolyzed, to get a precipitate, or slowly gelated under exposure to atmospheric humidity. The slow hydrolysis process was studied using nuclear magnetic resonance (NMR) spectroscopy in liquid phase. Two-dimensional experiments indicate the occurrence of ligand exchange reactions in solution. Monodimensional ¹H, ¹³C, ³¹P, and ²³Na NMR spectra were also measured. NMR allowed us to follow the advancement of the reaction and determine the degree of polymerization. NASICON powders having the Na₃Zr₂Si₂PO₁₂ composition were prepared by heating both precursors at various temperatures in the range 800-1200 °C for 1 h. The phase evolution as a function of the decomposition temperature was investigated by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) and X-ray diffraction (XRD) analysis. The NASICON prepared by fast hydrolysis contained a larger amount of free zirconia, while the samples prepared by slow hydrolysis were almost pure, showing a monoclinic structure.