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

The structure of as-synthesized TMA Silicate, an aluminosilicate synthesized with the tetramethyl ammonium (TMA) structure directing agent, was solved using intensities extracted from high-resolution synchrotron powder diffraction data and ab initio direct methods. The trial topology was improved by DLS refinement, and the structure was confirmed by 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 topology of as-synthesized TMA Silicate consists of 4-connected T-atoms in 4-, 5-, 6- and 8-rings. Two types of cages are interconnected by pores no larger than 6-rings. The larger cage is a peanut-shaped 30-hedron [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 structure 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 largest 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.