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 H-1, C-13, P-31, and Na-23 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 degrees 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.