The functionality of sensors or batteries relies in many cases on mobile ions such as Li⁺, Na⁺, O^2- or F^-, which are subjected to rapid self-diffusion processes. The dynamic parameters as well as the shape of the underlying motion correlation function depend on the dimensionality of the diffusion pathways. Model compounds help to unambiguously proof the concepts and models developed to describe low dimensional diffusion.^[1-3] The pseudo-binary fluoride RbSn₂F₅ with trigonal symmetry constitutes such a model substance.

Here, we present a new mechanochemical route to synthesise nanocrystalline RbSn₂F₅ via a one-pot reaction in a planetary mill. Soft annealing transforms the as prepared product into microcrystalline RbSn₂F_5. We used X-ray diffraction, thermal analyses and high-resolution ⁸⁷Rb, ¹⁹F , and ¹¹⁹Sn NMR spectroscopy to characterize structure and ion dynamics in detail. In particular, we took advantage of AC impedance spectroscopy and time-domain NMR measurements to record diffusion-induced spin-lattice-relaxation rates in both the laboratory and rotating frame of reference to monitor long-range as well as short-ranged F anion transport processes in RbSn₂F₅. It turned out that only an NMR spectral density function developed for 2D diffusion yields an activation energy being in accordance with that derived from conductivity measurements. Our results confirm Richards^[4,5] semi-empirical NMR relaxation model for 2D uncorrelated jump diffusion, not only for microcrystalline RbSn₂F₅ but also for the nanocrystalline, mechanosynthesized sample.

Financial support by the Austrian Federal Ministry for Science, Research and Economy as well as the Christian-Doppler Forschungsgesellschaft is highly appreciated.

References

[1] M. Wilkening, W. Küchler, and P. Heitjans, Phys. Rev. Lett. 97, 065901 (2006).

[2] A. F. McDowell, C. F. Mendelsohn, M. S. Conradi, R. C. Bowman, and A. J. Maeland, Phys. Rev. B.
51, 6336 (1995).

[3] V. Epp and M. Wilkening, Phys. Rev. B 82, 020301 (2010).

[4] P. M. Richards, Solid State Commun. 25, 1019 (1978).

[5] P. M. Richards. In Topics in Current Physics (Ed. M. B. Salamon), Band 15, Springer, Berlin (1979).