Solving centrosymmetrical zeolites from powder diffraction data by combining the direct-methods origin-free modulus sum function with the isomorphous replacement technique. X

A new two-stage strategy for the solution of zeolite crystal structures with centrosymmetrical frameworks is proposed; this strategy exploits the information contained in multiple powder diffraction patterns. In the first stage, the structure-directing agent (SDA), i.e. the template molecule, is located by combining the isomorphous replacement technique with direct methods. In the second stage, the framework atoms are found by applying direct methods strengthened with additional information coming from the located SDA. All phase refinements are based on the maximization of the origin-free modulus sum function as implemented in the direct-methods program XLENS [Rius (2004). Z. Kristallogr. 219, 826-832], with data resolution limits equal to 3.2 Å for the first and 2.21 Å for the second stage. The viability of this new two-stage strategy is illustrated with the solution of the novel zeolite ITQ-32, containing nine Si atoms in the asymmetric unit, from synchrotron powder diffraction data. Especially important is the confirmation that the intensity differences involved in the SDA isomorphous replacement are significantly greater than the experimentally measured errors.