Spatial confinement-rapid 2D F ^- diffusion in micro- and nanocrystalline RbSn ₂ F ₅

Diffusion of small ions in materials with confined space for translational dynamics can be quite different to isotropic (3D) diffusion, which is found in the majority of solids. Finding credible indications for 2D diffusion is not as easy as it looks at first glance, especially if only powder samples are available. Here we chose the ternary fluoride RbSn ₂ F ₅ as a new model system to seek out low-dimensional anion diffusion in a nanocrystalline material. We prepared RbSn ₂ F ₅ via mechanochemically-assisted solid state synthesis and used both ac conductivity spectroscopy and spin-lock NMR relaxation measurements to find evidence that the fluorine ions preferably diffuse between the Rb-rich layers. In both cases the diffusion induced spin-lock NMR rates are only consistent with conductivity data if they are analyzed with the semi-empirical spectral density function for 2D jump diffusion as introduced by P. M. Richards [Solid State Commun., 1978, 25, 1019].