THE TANGENT FORMULA DERIVED FROM PATTERSON FUNCTION ARGUMENTS: A USEFUL TOOL FOR SOLVING ZEOLITE STRUCTURES FROM X-RAY POWDER DATA. J. Rius & C. Miravitlles, Institut de Ciencia de Materials de Barcelona (CSIC), Campus de la UAB, 08193-Cerdanyola, Catalunya (Spain).

The viability of solving the structure type of zeolitic and layered materials applying multisolution direct methods to low resolution (2.2 Angstroms) powder diffraction data is shown. The phases are refined with the tangent formula derived from Patterson function arguments [Rius (1993). Acta Cryst. A49, 406-409] implemented in the XLENS program and the correct phase sets are discriminated with the conventional figures of merit.

The two test examples presented are (a) the already known tetragonal zeolite ZSM-11 (space group I-4m2) at 2.3 Angstroms resolution and (b) the hitherto unknown layer silicate RUB-15 (Ibam) at 2.2 Angstroms resolution. In both cases, the tetrahedral Si units appear as resolved peaks in the Fourier maps computed with the phases of the highest-ranked direct methods solutions.