Outcomes of the International Union of Crystallography Commission on Powder Diffraction round robin on quantitative phase analysis: samples 1a to 1h

The International Union of Crystallography (IUCr) Commission on Powder Diff raction (CPD) has sponsored a round robin on the determination of quantitat ive phase abundance from diffraction data. Specifically, the aims of the ro und robin were (i) to document the methods and strategies commonly employed in quantitative phase analysis (QPA), especially those involving powder di ffraction, (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 diffracti on data, and (v) to create a standard set of samples for future reference. Some of the analytical issues which have been addressed include (a) the typ e of analysis (integrated intensities or full-profile, Rietveld or full-pro file, database of observed patterns) and (b) the type of instrument used, i ncluding geometry and radiation (X-ray, neutron or synchrotron). While the samples used in the round robin covered a wide range of analytical complexi ty, this paper reports the results for only the sample 1 mixtures. Sample 1 is a simple three-phase system prepared with eight different compositions covering a wide range of abundance for each phase. The component phases wer e chosen to minimize sample-related problems, such as the degree of crystal linity, preferred orientation and microabsorption. However, these were stil l issues that needed to be addressed by the analysts. The results returned indicate a great deal of variation in the ability of the participating labo ratories to perform QPA of this simple three-component system. These differ ences result from such problems as (i) use of unsuitable reference intensit y ratios, (ii) errors in whole-pattern refinement software operation and in interpretation of results, (iii) operator errors in the use of the Rietvel d method, often arising from a lack of crystallographic understanding, and (iv) application of excessive microabsorption correction. Another major are a for concern is the calculation of errors in phase abundance determination , with wide variations in reported values between participants. Few details of methodology used to derive these errors were supplied and many particip ants provided no measure of error at all.