Quantitative analysis of diffraction data usually refers to determination of amounts of different phases in multi-phase samples by using standard sample or simulation method. Quantitative analysis may also be understood as determination of particular characteristics of single phases including precise determination of crystal grain size and its structure and shape.
There are several methods of X-ray diffraction to quantify phases, but the Rietveld method has been perhaps the most useful tool in recent years as it accounts for the factors that affect the reproducibility of the intensity peaks (peak overlapping, presence of amorphous phases, and preferred orientation of crystallites).
Ro Thae Min, a researcher at the General Assay Office, has observed the quantitative value change when lattice constant is used, and not used, for fitting in the quantitative phase analysis of TiO2 by the Rietveld method.
The conclusions are as follows.
In qualitative analysis, the corresponding phase may not be correctly identified or unreconciled lattice constant in the ICSD database may be used. To avoid such mistakes, in the quantitative phase analysis, correct lattice parameters must be used for fitting. When lattice parameters are correct, Rietveld method is better than other X-ray diffraction quantitative phase methods such as internal standard method and direct method because it employs insturments, structure parameters and lattice constant of analogue phases altogether. Sometimes, however, lattice constant is not used for fitting. In this case, various lattice defects (crystallite size, rms strain, stacking and twin fault probabilities, etc.) are found in the whole diffraction pattern improvement process. Therefore, in the quantitative phase by Rietveld method, lattice constant should not be ignored to get correct quantitative value.
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