Mobility of hydrated alkali metal ions in metallosupramolecular ionic crystals

The ion-conducting behaviour of alkali metal ions in ionic solids (M6[1]·nH2O) resembles that in aqueous solutions; the solid-state conductivities increase in the order of M = Li+ < Na+ < K+.
The design and creation of ionic crystals that show high mobility of ionic species in the solid state has long been a research topic of considerable attention not only due to the practical applications of these materials but also due to the correlation of such ionic species with ion-transport biological systems. In this work, we report the mobility of alkali metal ions (M = Li+, Na+, K+) in the ionic crystals M6[Rh4Zn4O (l-cysteinate)12]·nH2O (M6[1]·nH2O). In M6[1]·nH2O, alkali metal ions are distributed in a disordered manner, together with a number of water molecules, within a rigid hydrogen-bonded framework of anionic clusters of [1]6–. The alternating current conductivities of M6[1]·nH2O in the solid state increase in the order of Li6[1]·nH2O < Na6[1]·nH2O < K6[1]·nH2O, which is opposite to the order of the naked ionic radii. The conductivities reach the superionic level of σ = 1.3 × 10–2 S cm–1 at 300 K for a single crystal of K6[1]·nH2O. These results reflect the high mobility of hydrated alkali metal ions in the crystal lattice of M6[1]·nH2O, which is supported by solid-state NMR spectroscopy, together with ion diffusion experiments in the solid state. The high mobility leads to quick exchange of K+ ions in K6[1]·nH2O with Li+ and Na+ ions with retention of single crystallinity.