A TANGENT FORMULA DERIVED FROM PATTERSON-FUNCTION ARGUMENTS .3. STRUCTURE DETERMINATION OF ZEOLITIC AND LAYERED MATERIALS FROM LOW-RESOLUTION POWDER DIFFRACTION DATA

The viability of solving the structure type of zeolitic and layered ma terials applying multisolution direct methods to low-resolution (simil ar to 2.2 Angstrom) powder diffraction data is shown. The phases are r efined with the tangent formula derived from Patterson-function argume nts [Rius (1993). Acta Cryst. A49, 406-409] and the correct phase sets are discriminated with the conventional figures of merit. The two tes t examples presented are (a) the already known tetragonal zeolite ZSM- 11 (space group <I(4)over bar m2>) at 2.3 Angstrom resolution and (b) the hitherto unknown layer silicate RUB-15 (Ibam) at 2.2 Angstrom reso lution. In both cases, the tetrahedral Si units appear as resolved pea ks in the Fourier maps computed with the phases of the highest ranked direct-methods solutions.