Effects of composition and crystallite size on the accuracy of the Rietveld method in determining lattice parameters of polytypes in multiphase SiC ceramics

The accuracy of the Rietveld method in determining the lattice parameters of polytypes in multiphase SiC ceramics was investigated as a function of the polytype composition and of the polytype crystallite size, using as benchmarks two sets of standard X-ray diffraction (XRD) patterns obtained by computer simulation. It was found that the Rietveld method is remarkably accurate, but also that the measurement of lattice parameters is sensitive to the polytype composition and to the polytype crystallite size. In particular, the accuracy of Rietveld-calculated lattice parameters for a given polytype not only decreases with increasing number of polytypes in the SiC ceramics, but also depends on which other polytypes are present. These two findings are interpreted based on the severity of the corresponding peak overlap phenomenon in the XRD patterns. In addition, the accuracy of the Rietveld analyses also decreases with decreasing crystallite size of the polytypes, which is due to the progressive loss of definition in the location of the peak maxima and to the greater severity of the peak overlap, both resulting from the corresponding peak broadening. This identification of the confidence limits of the Rietveld-calculated lattice parameters for a wide range of microstructural features may have important implications for the future characterization of SiC ceramics by XRD.