IUCr activities

Powder Diffraction update

[Commission on Powder Diffraction]The Commission on Powder Diffraction (CPD) Newsletter now reaches 2000 recipients around the world. All the Newsletters since the first issue of 1989 will be published on CD-ROM soon. The congress on Accuracy in Powder Diffraction III (APD3, Gaithersburg, MD, USA, 22-25.47.2001) was a major event for the year 2001. Future meetings will include EPDIC 8 (Spring 2002) in Sweden and the IUCr World Congress (Geneva, August 2002) where eight microsymposia will be dedicated to powder diffraction and related topics and Structure solution and refinement will be central themes.

Issue No. 26 of the CPD Newsletter focused on structure determination from powder diffraction data. The field has developed rapidly and a diverse set of Crystallographic tools allow ab initio structure solution from powder diffraction data alone. Whilst not yet as routine as single crystal determinations, the application of structure determination from powder diffraction methods can be routine in many circumstances. Although computing power has catalysed many advances, especially in the area of global optimization, algorithmic developments, experimental ingenuity, chemical intuition and a refusal to accept the conclusion that certain things 'just can't be done using powder diffraction' have all contributed to recent successes.

Quantitative phase analysis round robin

The IUCr Commission on Powder Diffraction (CPD) sponsored a Round Robin on the determination of quantitative phase abundance from diffraction data. The aims of the Round Robin were: (i) to document the methods and strategies commonly employed in quantitive phase analysis, (ii) to assess levels of accuracy, precision and lower limits of detection, (iii) to identify specific problem areas and develop practical solutions, (iv) to formulate recommended procedures for QPA using diffraction data, and (v) to create a standard set of samples for future reference.

The preliminary report and analysis of results are available at www.boulder.nist.gov/div853/balzar, CPD and CCP14 websites.

Bill David, Paolo Scardi

Rietveld refinement

Issue No. 26 of the CPD newsletter is devoted to various aspects of modern Rietveld refinement. The basic idea behind the Rietveld method is to calculate the entire powder pattern using a variety of refinable parameters and to improve a selection of these parameters by minimizing the weighted sum of the squared differences between the observed and the calculated powder pattern using least squares methods. That way, the intrinsic problem of the powder diffraction methods with systematic and accidental peak overlap is overcome. Hugo Rietveld invented the method a few decades ago, to extract as much information as possible from powder patterns. Applications of the Rietveld method range from the refinement of small protein structures to quantitive phase analysis of industrial importance.

Robert Dinnebier