Abstract
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 M(6)[Rh(4)Zn(4)O (l-cysteinate)(12)]·nH(2)O (M(6)[1]·nH(2)O). In M(6)[1]·nH(2)O, 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 M(6)[1]·nH(2)O in the solid state increase in the order of Li(6)[1]·nH(2)O < Na(6)[1]·nH(2)O < K(6)[1]·nH(2)O, 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 K(6)[1]·nH(2)O. These results reflect the high mobility of hydrated alkali metal ions in the crystal lattice of M(6)[1]·nH(2)O, 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 K(6)[1]·nH(2)O with Li(+) and Na(+) ions with retention of single crystallinity.