H. M. Rietveld

Bio: H. M. Rietveld is an academic researcher from University of Western Australia. The author has contributed to research in topics: Rietveld refinement & Powder diffraction. The author has an hindex of 13, co-authored 17 publications receiving 16894 citations.

A profile refinement method for nuclear and magnetic structures

Abstract: A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.

Further refinement of the structure of WO3

Abstract: out to provide an estimate of the absorption correction. The ratio (maximum intensity/minimum intensity) was expected to be about 1.3 (UMo r= = 57 cm-1). Instead, we observed for 020, lm~/lma~ = 2\"56, and for 040, Im~/mln = 2\" 16. This deviation from the expected value, together with the clearly visible splitting of the reflexions into K~l-K~z-peaks at a 0 as low as 20 °, suggested that the crystal was almost perfect, and that the intensities were affected by primary extinction. The same was also observed in the structure analyses of the MNb3S6 series ( M = M n , Fe, Co, Ni; Anzenhofer, van den Berg & Helle, 1968) and in this case a correction procedure was applied which led to very good results. This therefore encouraged us to use the same correction procedure for the CoMo2S4 intensity data. Essentially this meant an extrapolation of the intensities, 1, to values which would have been measured at the azimuth angle, where I=lm~. The correction factor was derived from the azimuthal scans. The refinement was started in C2/m with the structure model derived by van den Berg (1968). After a few cycles, the conventional R index had dropped to 16.2%. From this point onwards 14 poorly determined structure factors were omitted; also individual anisotropic temperature factors were introduced into the subsequent refinement cycles, which finally brought R down to 8.7%. The next problem was to verify the space group C2/m. We modified the structure parameters to meet the specific symmetry requirements of Cm and C2, respectively, and refined both models. Both models clearly moved towards the C2/m model, finally terminating with a singularity in the matrix when the two models approached values of structure parameters which are fixed in C2/m. So the space group was verified as C2/m. The final structure parameters are presented in Table 1 together with the estimated standard deviations. Fig.2(a) and (b) s, hows the distorted sulphur octahedra about the Co and Mo atoms respectively. Bond lengths and angles are included to give the shape and distort ion of the octahedra. A discussion of the CoMozS4 structure and its relation to the magnetic properties has been presented by van den Berg (1968). A list of the structure factors involved in the refinement calculations may be obtained on request.

The Rietveld method

Abstract: A summary is given of the development of an improved method to handle powder diffraction data. The resulting method, now called the Rietveld method, uses powder diffraction step-scanned intensities instead of integrated powder intensities. This enables the full use of the information content of a powder diagram. The method has revitalized the use of powder diffraction in structure determination. An unexpected and later development is the use of the method in quantitative phase analysis. This is now an essential tool in many industrial processes.

Mechanochemistry: opportunities for new and cleaner synthesis

Abstract: The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

Fityk: a general-purpose peak fitting program

Abstract: Fityk is portable, open-source software for nonlinear curve fitting and data analysis. It specializes in fitting a sum of bell-shaped functions to experimental data. In particular, it enables Pawley refinement of powder diffraction data and size–strain analysis.

Crystalline Polymorphism of Organic Compounds

Abstract: Crystal polymorphism is encountered in all areas of research involving solid substances. Its occurrence introduces complications during manufacturing processes and adds another dimension to the complexity of designing materials with specific properties. Research on polymorphism is fraught with unique difficulties due to the subtlety of polymorphic transformations and the inadvertent formation of pseudopolymorphs. In this report, a summary of thermodynamic, kinetic and structural considerations of polymorphism is presented. A wide variety of techniques appropriate to the study of organic crystalline polymorphism and pseu-dopolymorphism is then surveyed, ranging from simple crystal density measurement to observation of polymorphic transformations using variable-temperature synchrotron X-ray diffraction methods. Application of newer methodology described in this report is yielding fresh insights into the nature of the crystallization process, holding promise for a deeper understanding of the phenomenon of polymorphism and its practical control.