Quantitative polytype-composition analyses of SiC using X-ray diffraction:a critical comparison between the polymorphic and the Rietveld methods

X-ray powder diffraction (XRD), in conjunction with an analysis routine. is the method of choice for performing quantitative phase-composition measure ments in multi-phase mixtures. However, accurate determination of phase com positions in SiC-based ceramics, which can contain a multitude of SiC polyt ypes with overlapping Bragg reflections, is a difficult task. In order to d etermine the best analysis method for the quantitative phase-composition me asurement in SiC-based ceramics, a critical comparison between two commonly used analysis methods - polymorphic method (Ruska) and whole pattern metho d ( Rietveld) - was performed. Owing to the difficulty in obtaining high-pu rity standard mixtures of SiC poly types experimentally, we have simulated a set of XRD patterns corresponding to different SiC polytype mixtures of w ide ranging compositions as standards. Within the polymorpic method, the Fo llowing three different approaches were used; (i) raw XRD peak heights were obtained and corrected for mean background level; (ii) a non-linear Levenb erg-Marquardt least-squares fit was used to obtain peak heights: and (iii) using the same fitting procedure as in (ii) integrated intensities were obt ained. We demonstrate that the Rietveld method yields the most accurate pha se-composition measurements. with mean and maximum errors of 0.5 wt.% and 0 .9 wt.%. respectively. In the polymorphic method, we show that a fitting pr ocedure is essential for the improvement in the accuracy of the analysis. F urthermore, we find that the use of integrated intensities, obtained from t he fit, for quantitative XRD analyses gives results that are less accurate compared with when corrected raw peak heights are used. (C) 2001 Elsevier S cience Ltd. All rights reserved.