NMR investigations of Li+ ion dynamics in the NASICON ionic conductors

NMR studies of 7Li and 31P nuclei are reported in the 150–900 K temperature range for the NASICON compounds with x = 0.8, 0.7, 0.6 and 0.3. Magic angle spinning (MAS mode) experiments were performed at room temperature on the 7Li and 31P nuclei. The linewidth and the spin lattice relaxation times of these nuclei are studied versus temperature in the static mode. The spectra recorded in the MAS mode show that the 7Li ions occupy three chemical sites, the occupation of which being very sensitive to the x values but not sensitive to the coexistence of the two varieties and observed at room temperature in compounds with x≤0.5. On the other hand, the 31P nucleus MAS spectra are very sensitive to lithium content but also to the variety coexistence. T1 measurements were performed in a static mode on the 7Li and 31P nuclei. In all the compounds, the 7Li spin lattice relaxation time exhibits two branches with several minima, indicating the complex dynamics for this ion. One of these minima appears in the same temperature range as the minimum of the 31P nucleus T1, strongly suggesting a cross-relaxation process between these nuclei. T1ρ measurements on 7Li (static mode) allow us to show a slow motion different from the one probed by the T1. The analysis of the T1ρ behaviour with temperature and composition allows us to ascribe the motion probed by this time to the oxygen ion motion which monitors the opening and closing of the lithium pathways. A qualitative interpretation of the 7Li T1 results is done; it takes into account the cross-relaxation phenomena between 31P and 7Li and quadrupolar fluctuations.