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The Background in X-Ray Powder Diffractograms: A Case Study of Rietveld Analysis of Minor Phases Using Ni-Filtered and Graphite-Monochromated Radiation

Published online by Cambridge University Press:  10 January 2013

R.J. Hill
R.J. Hill
Affiliation:
CSIRO Division of Mineral Products, P.O. Box 124, Port Melbourne, Australia
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Abstract

High resolution X-ray powder diffraction data have been collected with Bragg-Brentano geometry on samples of MgO using Ni-filtered and graphite-monochromated CuKαradiation. Selection of the characteristic radiation by Ni-filtering produces severe peak asymmetry, truncates the low-angle foot of the peak, lowers the general level of background on the low angle side, and leaves a remnant Kβpeak for all foils of reasonable thickness. When step-scan data produced by this method are used for Rietveld analysis, all of these features cause difficulties in fitting a smooth function to the background and in successfully modelling the detailed profiles of the peaks. On the other hand, Kαradiation from a diffracted-beam monochromator provides inherently more symmetric peaks and a smoothly varying background on both sides of the peak centre, both of which effects can be adequately modelled during Rietveld analysis. The primary disadvantage with monochromation is that, even with very careful setting of the pulse height discrimination, the monochromator may pass a small proportion of the λ/2 component of the incident radiation. In samples containing small quantities (i.e., 2 wt%, or less) of impurity phases, the undesirable features of the diffractometer profile (i.e., asymmetric and truncated background, and Kβand λ/2 peaks) can be of similar intensity to the main peaks arising from the impurities (as well as substructure peaks from the primary phases), thereby leading to difficulties in their identification and quantification. Nevertheless, with due care and long data collection times, the abundances of minor phases can be measured with Rietveld analysis down to levels of the order of 0.1 wt%.

Type
Research Article
Information
Powder Diffraction , Volume 7 , Issue 2 , June 1992 , pp. 63 - 70
Copyright
Copyright © Cambridge University Press 1992

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