Ion Mobility in Triple Sodium Molybdates and Tungstates with a NASICON Structure

Research data for the diffusion mechanisms of Na + ions in Na 1 – x Mg 1 – x Al 1 + x (XO 4 ) 3 (X = Mo, W) compounds with the NASICON-type structure (space group R c , Z = 6) are reported. Solid solutions in the homogeneity range 0.1 ≤ x ≤ 0.5 for X = Mo and 0.4 ≤ x ≤ 0.6 for X = W have been prepared by solid-state synthesis. Conductivity measurements and NMR spectroscopy data indicate fast sodium diffusion in the studied samples: the ionic conductivity reaches the values of about 10 –3 S/cm at T > 800 K. The frequency of elementary ionic jumps is on the order of 10 4 s –1 at T ≈ 500 K, and the activation energy is equal to 0.8–0.9 eV. The results have shown that the ionic conductivity in molybdates is higher than in tungstates. The growth of magnesium concentration increases the concentration of local coordinations Mg 2+ –Na + –Mg 2+ , acting as traps for moving sodium ions. The above conclusions are supported by ab initio calculations according to which the barrier for sodium diffusion from the Mg 2+ –Na + –Mg 2+ position is expected to be higher than those for the Mg 2+ –Na + –Al 3+ and Al 3+ –Na + –Al 3+ ones.