Crystal structure of a carbonate-nosean Na8[AlSiO4]6CO3

The crystal structure of a nosean-analogous, carbonate-enclathrated compound of ideal composition Na-8[AlSiO4](6)CO3 (cubic, a(0) = 900.1(1) pm) was determined from powder data using the Rietveld method. The sample was prepared by annealing a basic nitrite sodalite solid solution in a Stream of carbon dioxide (4,6 l/h) at elevated temperatures (873 K and 1023 K). The total ordering of the framework atoms Si(OAl)(4) was revealed by a MAS NMR investigation of Si-29. In contrast to the sodalite structure, the cage-filling clusters [Na4CO3](2+) and [Na-4](4+) are fully ordered. Space group P23 is confirmed along with the ordering of framework atoms. In accordance with the difference in size between the ordered guest ions, two independent positions for the framework oxygen atoms were found, each associated with a lar er or a smaller cage type. These special features of the framework can also be ascertained from the split of the v(s)-mode in the 730 cm(-1) region of the symmetric vibrations of the framework constituents in the IR spectra. The carbonate anion, with the carbon atom located at the central position, exhibits 12-fold orientational disorder within the beta-cage. The sodium cations are distributed over two positions. From their coordinates, it is possible to derive the location of the cations inside the anion-free cages very close to the six-membered rings of the aluminosilicate framework. The configuration of the ions inside both types of cages should not be considered as static but rather on the basis of highly dynamical behaviour which leads to an optimal charge distribution between anion-containing cages and anion-free cages.