Lithium dynamics in the zeolite-like lithosilicate RUB-29 and its high-temperature structure after dehydration (Cs₁₄Li₄₂Si₇₂O₁₇₂)

The thermally induced modifications of the zeolite-like lithosilicate RUB-29 (Cs₁₄Li₂₄[Li₁₈Si₇₂O₁₇₂] · 14H₂O, space group I222, a = 11.208(1), b = 17.286(1), c = 23.536(1) Å, and V = 4660(1) ų [S.-H. Park, J.B. Parise, H. Gies, H. Liu, C.P. Grey, B.H. Toby, J. Am. Chem. Soc. 122 (44) (2000) 11023-11024]) have been studied focusing on static and dynamical disorder of Li. After the water loss, dehydrated RUB-29 (HT-d-RUB-29) maintains the basic space group I222 at high temperatures from 473 up to 1073 K. However, the distribution of extra-framework Li and Cs cations in HT-d-RUB-29 became distinctly different from those in the original structure. In particular, extra-framework Li cations located within porous 8- and 10-membered ring-channels, where these Li cations were partially coordinated with zeolitic water in the original structure, migrate into densely packed Li₂O-layers after dehydration. As a result of the migration, the number of empty sites relevant for Li hopping in Li₂O-layers decreases, which give a negative effect on the long-range charge transfer in dehydrated RUB-29. Its DC conductivity values estimated from AC impedance spectra lie between 2 × 10^-5 and 6 × 10^-5 S cm^-1 at 873 K, and a low activation energy of 51 kJ/mol (≅ 0.53 eV) could be determined for the bulk charge transfer process in dehydrated RUB-29.