DIRECT-METHODS STRUCTURE DETERMINATION FROM SYNCHROTRON POWDER DIFFRACTION DATA OF A NEW CLATHRASIL, TMA SILICATE

The structure of as-synthesized TMA Silicate, an aluminosilicate synth esized with the tetramethyl ammonium (TMA) structure directing agent, was solved using intensities extracted from high-resolution synchrotro n powder diffraction data and ab initio direct methods. The trial topo logy was improved by DLS refinement, and the structure was confirmed b y successful Rietveld refinement. The space group is C2/m, and lattice parameters are a = 13.35211(14) Angstrom; b = 13.05531(12) Angstrom; c = 12.53013(13) Angstrom; beta = 113.285(1)degrees. The framework top ology of as-synthesized TMA Silicate consists of 4-connected T-atoms i n 4-, 5-, 6- and 8-rings. Two types of cages are interconnected by por es no larger than 6-rings. The larger cage is a peanut-shaped 30-hedro n [4(8)5(12)6(10)8] that is the fundamental polyhedral building unit. The three-dimensional structure results from sharing the 4-, 5- and 6- ring faces between adjoining large cages. The three-dimensional struct ure also contains a smaller 10-hedron cage [4(4)5(4)6(2)]. Both cages have crystallographic 2/m site symmetry. Tetramethyl ammonium cations are located in each lobe of the peanut-shaped cages. Because the large st opening to any cage is through 6-ring pores, which are too small to let the TMA pass, as-synthesized TMA Silicate can be classified as a clathrasil.