engleski

Line-profile analysis and standards

It was recognized early on that crystal defects cause broadening of diffraction lines. Thus, the analysis of line broadening can yield details of crystalline imperfections and associate strains at near-atomic resolution. Yet, much of the related research has stopped at a qualitative assessment for at least two reasons: the lack of comprehensive physical models and difficulties in characterizing different origins of line broadening, including the instrument contribution. As physical-broadening effects in the specimen decrease, the accurate characterization of instrumental broadening becomes increasingly important. The contribution of optical elements to line broadening in a particular diffractometer can be calculated by several methods. Nevertheless, there will always be a need to compare different instruments using a reference specimen. Moreover, the inability to account for the misalignment or imperfection of some of the optical elements may present difficulties. Therefore, there is a continuing need for direct measurement of instrumental line broadening using suitable standard materials. Several NIST reference materials will be compared. Line-profile analysis has traditionally been performed with line-profile fits of individual lines. In a whole-powder-pattern-fitting approach, such as in Rietveld refinement, where the focus is on determination of structural parameters, line-broadening effects are usually treated as a nuisance. The preferred approach is to include refinable parameters with a physical significance within accurate line-broadening models. An accurate description of diffraction-line profiles, modeling of the angular dependence of line broadening, and anisotropic effects then become of the utmost importance. Some novel approaches will be discussed and applications to several materials described.

X-ray powder diffraction. crystal deffects. line broadening. diffraction-line profiles

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