CRYSTAL-STRUCTURE OF A CARBONATE-NOSEAN NA-8[ALSIO4](6)CO3

The crystal structure of a nosean-analogous, carbonate-enclathrated co mpound 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 sa mple 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) w as revealed by a MAS NMR investigation of Si-29. In contrast to the so dalite structure, the cage-filling clusters [Na4CO3](2+) and [Na-4](4) are fully ordered. Space group P23 is confirmed along with the order ing of framework atoms. In accordance with the difference in size betw een the ordered guest ions, two independent positions for the framewor k oxygen atoms were found, each associated with a lar er or a smaller cage type. These special features of the framework can also be ascerta ined from the split of the v(s)-mode in the 730 cm(-1) region of the s ymmetric vibrations of the framework constituents in the IR spectra. T he carbonate anion, with the carbon atom located at the central positi on, exhibits 12-fold orientational disorder within the beta-cage. The sodium cations are distributed over two positions. From their coordina tes, it is possible to derive the location of the cations inside the a nion-free cages very close to the six-membered rings of the aluminosil icate framework. The configuration of the ions inside both types of ca ges should not be considered as static but rather on the basis of high ly dynamical behaviour which leads to an optimal charge distribution b etween anion-containing cages and anion-free cages.