A tangent formula derived from Patterson-function arguments. Pt. 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 materials applying multisolution direct methods to low-resolution (∼2.2 A) powder diffraction data is shown. The phases are refined with the tangent formula derived from Patterson-function arguments 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 anti 4m2) at 2.3 A resolution and (b) the hitherto unknown layer silicate RUB-15 (Ibam) at 2.2 A 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. (orig.)