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The structure of the calcined borosilicate zeolite catalyst SSZ-82 ([Si61.3B4.7O132], Pmmn, a = 24.2783(4), b = 11.4665(2), and c = 14.1127(3) Å) has been solved from X-ray powder diffraction (XPD) data using the recently developed 2D-XPD charge flipping approach. The electron density maps generated with the more conventional powder charge flipping (pCF) algorithm could not be interpreted easily, so this new method, which begins by phasing low-resolution, 2D subsets of the data, was applied. Crystallographic phases were derived for the three main projections ([100], [010], and [001]) by using just the corresponding subsets of reflections (0kl, h0l, and hk0, respectively) from the full set of 3039 extracted intensities. These phases were then imposed on the (otherwise random) starting phases in the application of the pCF algorithm to the full data set. The framework structure, with 11 Si/B atoms in the asymmetric unit and a novel 12-/10-ring 2D channel system, could be seen clearly in the resulting electron density map. This is the first application of the 2D-XPD method to data collected on a material of unknown structure. Rietveld refinement of the structure revealed the positions of the B atoms in the framework and indicated that some water had been readsorbed in the pores. [ABSTRACT FROM AUTHOR]

Abstract: The zeolite framework structure of the germanosilicate SSZ-77 ([Si45.7Ge10.3O112], C2/m, a =13.2090(3)Å, b =13.3238(3)Å, c =21.5551(5)Å, β =123(2)°) has been re-examined using high-resolution synchrotron powder diffraction data collected with better counting statistics at higher angles. In the original Rietveld refinement, the geometry was strained and not all of the Ge atoms could be located. With the better data, both of these problems could be resolved. The powder charge-flipping algorithm in the program Superflip was used to determine that the space group should be reduced from C2/m to C2. With this symmetry reduction, the Ge atoms could be located more correctly, and the geometry became less strained. In the final structure, one of the 14 T sites (tetrahedrally coordinated atoms) is pure Ge, four are mixed Ge/Si positions, and the remaining nine are pure Si. Diffuse scattering under the Bragg peaks is clearly evident in the background correction. This is consistent with a Si/Ge substitutional disorder in which there is a positive correlation between neighboring unit cells. That is, adjacent unit cells are more likely to have the same ordering, so the crystal contains domains of similar unit cells rather than a completely random ordering from unit cell to unit cell. [Copyright &y& Elsevier]

Physico-chemical characterization of the high-silica zeolite catalyst SSZ-74 (ref. 1) suggested that it, like the related materials TNU-9 (ref. 2) and IM-5 (ref. 3), has a multidimensional 10-ring channel system. Such pore systems are ideal for many petrochemical applications, and indeed SSZ-74 has been shown to be a good catalyst for a wide variety of reactions. The elucidation of its framework structure, however, proved to be difficult. Comparable problems were encountered with TNU-9 and IM-5, which were synthesized with related structure-directing agents. Their framework structures, which are the two most complex ones known, both have 24 Si atoms in the asymmetric unit, and were finally solved by combining high-resolution powder diffraction data with information derived from high-resolution electron microscopy images. Therefore, a similar approach, using the powder charge-flipping algorithm to combine the two types of data and molecular modelling to help to locate the structure-directing agent, was applied to SSZ-74. This procedure eventually revealed a most unusual 23-Si-atom framework structure (|(C16H34N2)4&Si92□4O184(OH)8]) with ordered Si vacancies. [ABSTRACT FROM AUTHOR]

Examines the use of structure envelopes in structure determination from powder diffraction data. Combination of the structure with direct-space methods to facilitate structure solution from powder data; Generation of an informative structure envelope; Assessment of the extension to molecular structures in combination with a simulated-annealing program.

The article discusses the new algorithm in determining crystal structures from diffraction data. Introduced in 2004, the algorithm was based on a very simple perturbation, called charge flipping, of electron-density maps that are generated during the structure solution process. Crystallographers tested the algorithm and the result is that the approach has found niches in areas in which traditional methods flounder.

Abstract: In an attempt to prepare a novel condensed silicate containing double 4-rings, a hydrothermal synthesis using a solution of octakis(tetramethylammonium) octasilsesquioxane hydrate as the starting material was performed. The product (DLM-2) was more thermally stable than the precursor, indicating that at least some condensation had taken place, but the solution of the structure of this polycrystalline material (∣((CH3)3N)8 (H2O)20∣[Si24O48(OH)8]) turned out to be more difficult than expected. This was primarily a result of the fact that the correct symmetry was unclear. However, the structure was finally solved from powder diffraction data collected on a textured sample using the powder charge flipping algorithm pCF in the program Superflip. This algorithm is symmetry independent, so the space group problem could be circumvented. The structure proved to be non-centrosymmetric (Ima2, a =11.5243(1)Å, b =8.4136(1)Å, c =27.9075(4)Å) and consists of silicate layers hydrogen-bonded to one another via water molecules with TMA+ cations encapsulated in the voids. However, there are no double 4-rings in the structure, suggesting that softer reaction conditions are needed for them to retain their structural integrity during the synthesis. The structure is very similar to that of the zeolite precursor RUB-15. [Copyright &y& Elsevier]

Presents a report on the development of a method for solving structures from powder diffraction data. Demonstration of validity on three complex structures; Use of a textured sample that exploits the high intensity and parallel nature of synchrotron radiation; The principle of crystal structures; Results.

A new medium-pore germanosilicate, denoted IM-18, with a three-dimensional 8 × 8 × 10-ring channel system, has been prepared hydrothermally using 4-dimethylaminopyridine as an organic structure-directing agent (OSDA). Due to the presence of stacking disorder, the structure elucidation of IM-18 was challenging, and a combination of different techniques, including electron diffraction, high-resolution transmission electron microscopy (HRTEM), and Rietveld refinement using synchrotron powder diffraction data, was necessary to elucidate the details of the structure and to understand the nature of the disorder. Rotation electron diffraction data were used to determine the average structure of IM-18, HRTEM images to characterize the stacking disorder, and Rietveld refinement to locate the Ge in the framework and the OSDA occluded in the channels. [ABSTRACT FROM AUTHOR]