Rietveld refinement guidelines McCusker, L. B.; Von Dreele, R. B.; Cox, D. E. ...
Journal of applied crystallography,
February 1999, Letnik:
32, Številka:
1
Journal Article
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A set of general guidelines for structure refinement using the Rietveld (whole‐profile) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction. The ...practical rather than the theoretical aspects of each step in a typical Rietveld refinement are discussed with a view to guiding newcomers in the field. The focus is on X‐ray powder diffraction data collected on a laboratory instrument, but features specific to data from neutron (both constant‐wavelength and time‐of‐flight) and synchrotron radiation sources are also addressed. The topics covered include (i) data collection, (ii) background contribution, (iii) peak‐shape function, (iv) refinement of profile parameters, (v) Fourier analysis with powder diffraction data, (vi) refinement of structural parameters, (vii) use of geometric restraints, (viii) calculation of e.s.d.'s, (ix) interpretation of R values and (x) some common problems and possible solutions.
The germanosilicate ITQ-24 (IWR framework type) was synthesized in fluoride medium using 1,3,5-tris(1,2-dimethylimidazolium) benzene as the structure directing agent (SDA). A structure analysis of ...the as-synthesized ITQ-24 material using synchrotron powder diffraction data and difference electron density calculations have allowed the fluoride ions and the germanium atoms to be located and the conformation of the SDA to be determined. The benzyl ring is perpendicular to the b axis with the three imidazolium moieties forming a "T-shaped" arrangement. Ge atoms replace some of the Si in the double-4-ring (d4r) and in one of the single-4-rings (s4r). The other s4r contains only Si. Fluoride ions are in the d4r units. Initially, the space group Cmmm (highest possible symmetry) was assumed, but the framework geometry was strained. An independent evaluation of the symmetry using the powder charge flipping algorithm in Superflip led to a successful refinement with reasonable geometry and a refined composition of |(C6H3)(C7H10N2)32F2|Si(40.2)Ge(15.8)O112 in the space group Pban.
A system of terms applicable to ordered microporous and mesoporous materials is proposed, and rules for writing a standardized crystal chemical formula for such materials are presented. The ...recommendations are based both on common usage and on a systematic classification scheme. The nomenclature has been developed to encompass all inorganic materials with ordered, accessible pores with free diameters of less than 50 nm. The crystal chemical formula describes the chemical composition of both the guest species and the host, the structure of the host, the structure of the pore system, and the symmetry of the material. This formula can be simplified or expanded to suit the user's requirements.
The structure of a new cubic gallophosphate with a pore opening comprising 20 tetrahedrally coordinated atoms in the shape of a four-leafed clover and a three-dimensional channel system is reported.
The FOCUS method, in which both crystal chemical information and powder diffraction data are included in the structure determination process, is presented. FOCUS combines automatic Fourier recycling ...with a specialized topology search specific to zeolites, which can be described as having three‐dimensional four‐connected framework structures. The capabilities of FOCUS have been tested with seven examples of medium to high complexity. The method was then applied to three novel zeolite structures and a promising model could be obtained in each case. Experience shows that the approach of using chemical and geometric knowledge can compensate for some of the information that is lost as a result of the overlap problem. At the same time, there is an intrinsic disadvantage: any method based on assumptions of certain structural properties is also limited to materials which conform to these assumptions. Examples which show the consequences of relaxing the structural assumptions are also given.
The FOCUS approach to zeolite structure determination from powder diffraction data has been applied to data from four different zeolitic materials. The solutions of the structures of two ...aluminophosphate molecular sieves, YUL‐89 (AWO topology) and YUL‐90 (ZON topology), are used to demonstrate routine applications of the procedure. The high‐silica zeolite ZSM‐5 (MFI topology), which has 12 Si atoms (38 framework atoms) in the asymmetric unit, and the gallophosphate cloverite (‐CLO topology), the framework of which is not fully fourfold connected, provide examples of extreme cases, which challenge the limits of the FOCUS algorithm. Taken together, the four examples give an overview of the practical aspects of the FOCUS method and illustrate its potential and its limitations.
The art of solving a structure from powder diffraction data has developed rapidly over the last ten years to the point where numerous crystal structures, both organic and inorganic, have been solved ...directly from powder data. However, it is still an art and, in contrast to its single crystal equivalent, is far from routine. The art lies not only in the correct application of a specific experimental technique or computer program, but also in the selection of the optimal path for the problem at hand.
A BEC-type zeolite (polymorph C of zeolite beta) with low germanium content (Si : Ge = 5.1) has been synthesised using the flexible linear diquaternary cationic form of pentamethyldiethylenetriamine ...(2,2'-(methylazanediyl)bis(N,N,N-trimethylethanammonium ) as the organic structure-directing agent (SDA). The distribution of germanium within the framework and the location of the SDA within the pores have been determined by analysing synchrotron X-ray powder diffraction data collected on the as-synthesised form. The findings, which are corroborated by super(13)C and super(19)F NMR data, indicate that the structure of the SDA is retained during the synthesis and that the Ge atoms occupy only sites in the double 4-ring. Although the BEC framework structure has a three-dimensional 12-ring channel system, the SDA molecules are found to orient themselves in two dimensions along the channels running parallel to the aand baxes.
Given just a few strong low‐index reflections, a periodic nodal surface (PNS), which divides the unit cell into regions of high and low electron density, can be generated. Fourteen known zeolite ...structures were examined to verify the validity of the procedure developed and in all cases the framework atoms were found to lie on just one side of the calculated curved surface. In other words, the surface defines a structure envelope. The same approach was applied to a few ionic and organic structures and was found to be equally valid. Since these reflections, with large d spacings, are precisely the ones that are least likely to be involved in overlap in a powder diffraction pattern, such a PNS can also be calculated using powder data. The resulting restriction in the region of the asymmetric unit in which atoms are likely to be found has immediate implications for any of the direct‐space methods of structure determination from powder data. The use of this structure envelope as a mask in a straightforward grid search procedure reduced the computer time required to solve several framework structures by as much as two orders of magnitude.
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