Theab initiostructure determination of Sigma-2 (a new clathrasil phase) from synchrotron powder diffraction data

The structure of Sigma-2 ([Si64O1 28].4C10H17N), a new clathrasil phase, has been determined directly from synchrotron X-ray powder diffraction data using no prior structural information. The high-resolution data were collected at the NSLS in Brookhaven, and then processed through a series of computer programs assembled and modified for this purpose. There are four Si and seven O atoms in the asymmetric unit, and nine of the 11 framework-atom positions appeared in the direct-methods solution. Eight of these were used to generate a Fourier map which clearly showed the remaining three oxygen positions. A whole-pattern refinement in the space group I41/amd [a = 10.2387(1) and c = 34.3829(1) Å] converged with RF = 0.100, Rwp = 0.225 and Rexp = 0.190. The designation SGT for this new zeolite structure type has been approved by the IZA Structure Commission. Neither of the two cages which make up Sigma-2 has been encountered previously. The 1-aminoadamantane used in the synthesis was found to be disordered in the large cage. The JCPDS Diffraction File No. is PDF39-2000.