Increase of ionic conductivity in the microporous lithosilicate RUB-29 by Na-ion exchange processes

The ionic conductivity in the zeolite-like lithosilicate RUB-29 (Cs₁₄Li₂₄Li₁₈Si₇₂O₁₇₂]·14H₂O S.-H. Park, J.B. Parise, H. Gies, H. Liu, C.P. Grey, B.H. Toby, J. Am. Chem. Soc. 122 (2000) 11023–11024]) increases via simple ion-exchange processes, in particular when Na cations replace a part of Cs⁺ and Li⁺ of the material. The resulting ionic conductivity value of 3.2×10⁻³ S cm⁻¹ at 885 K is about two orders higher than that for the original material S.-H. Park, J.B. Parise, M.E. Franke, T. Seydel, C. Paulmann, Micropor. Mesopor. Mater., in print (doi:10.1016/j.micromeso.2007.03.040 available online since April 19, 2007)]. The structural basis of a Na⁺-exchanged RUB-29 sample (Na-RUB-29) at 673 K could be elucidated by means of neutron powder diffraction. Rietveld refinements confirmed the replacement of Na⁺ for both parts of Cs and Li cations, agreeing with idealized cell content, Na₈Cs₈Li₄₀Si₇₂O₁₇₂. As a result of the incorporation of Na⁺ in large pores, the number of Li⁺ vacancies in dense Li₂O-layers of the structure could increase. This can be one of the main reasons for the improved conductivity in Na-RUB-29. In addition, mobile Na cations may also contribute to the conductivity in Na-RUB-29 as continuous scattering length densities were found around the sites for Na in difference Fourier map.