Search

Structural determination using X-ray powder diffraction is usually restricted to relatively simple structures, although powder diffraction data are frequently used to refine, via the Rietveld method, approximately known structures. In this work, the relatively complex crystalline structure of a compound of high pharmaceutical interest (C11H12N4S2) has been firstly ab initio determined from its X-ray powder diffraction pattern and subsequently refined by the Rietveld method. This refined structure is in excellent agreement with that obtained previously from single-crystal data. As a consequence of the severe overlapping between adjacent reflections, direct methods failed, whereas we obtained good outcomes with ‘real space’ methods. This work shows that within the current state-of-art (hardware, software, numerical methods, etc.), relatively complex structures can be afforded by means of powder diffraction.

Accurate X-ray powder diffraction (XRD) analysis of SiC-based ceramics is a difficult task due to the significant overlap of the Bragg reflections from the different SiC polytypes. For this reason, results obtained by traditional XRD methods are, in general, unsatisfactory. Here, we have applied the Rietveld and two line-broadening (variance and integral breadth) methods to analyze a liquid phase-sintered SiC sample. The Rietveld method was used to carry out the quantitative phase-composition analysis and to resolve (as a first step) the overlapped SiC Bragg reflections. Starting from the Rietveld results, the different SiC peaks were then unambiguously obtained by a Levenberg–Marquardt non-linear least-squares fit. Subsequently, the variance and integral breadth methods were used to perform the microstructural analysis. It is shown not only that the procedure is especially useful, but also that both line-broadening methods lead to similar crystallite sizes. The co-existence of β (3C) and α (4H and 6H) SiC polytypes and the nanometric diffraction domain sizes for the SiC polytypes were also found. Finally, these results are discussed in the light of the β to α partial transformation that took place during the sintering.

A quantitative determination of the phase abundance on Y-PSZ (4 mol%) polycrystals is performed by X-ray diffraction (XRD). Our procedure takes account of the strong peak overlapping, the presence of t′ phase and preferred orientation artifacts. The individual reflections of each phase are fitted to Pearson VII functions. The influence of the preferred orientation is handled by a procedure in which an expansion in spherical harmonics allows us to relate the measured integrated intensities to the “randomoriented” ones. The method is used to study the microstructural evolution of two sintered samples (1550°C and 1650°C) heat treated in air at 1600°C and subsequently air cooled.