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Rietveld Analysis: Choosing X-ray Powder Diffraction Data Obtained from Parallel and Focusing Beam Geometry, Aklilesh Tripathi, Tom Ely, Lori Fields, and Tom McNulty, Rigaku Americas Corporation, The Woodlands, TX
Typically, in conventional unit cell parameter determinations that utlize data collected in Bragg Brentano para-focusing beam mode, the systematic errors in peak positions such as those resulting from zero point shift, sample displacement, roughness and transparency are compensated for, at least in part, by using an internal standard. However, a major drawback to this approach is the contamination of the sample by the internal standard. This is not a desirable prospect particularly if one is working with a small quantity of a rare mineral or reactive material. Also, for complex powder patterns, it can be challenging to identify an internal standard that yields sufficient well resolved peaks with no overlaps to permit an accurate correction. In this paper, Rietveld refinement results are presented on the X-ray powder diffraction data collected with four standard reference materials and two other materials: acetaminophen and a rock sample composed of six mineral phases. The data has been collected in both parallel beam and para-focusing beam geometry. The results of the Rietveld refinement clearly indicate that the careful choice of parallel and/or focused beam geometry for a particular sample type gives better agreement factors over a situation wherein only one type of geometry is present.