The accuracy attainable nowadays in recording diffraction patterns is in fact incompatible with the imposition of a certain profile shape on subsequent line-broadening analysis. Thus, recognizing that the power of total pattern analysis on the basis of an assumed profile shape relies on a relatively crude correction for overlap of reflections, possibly as part of the Rietveld method for refinement of the idealized crystal structure, nevertheless analysis of crystal imperfection along this route is a dead-end street. This comment, extracted from the preface for EPDIC 3 by Delhez and Mittemeijer [1], focus exactly on the title subject.
The present review (based on [2] and references therein) relates how and why were explored some routes leading to that " dead-end street. Accounting for size-microstrain in whole powder pattern fitting (WPPF) may consist simply of the extension of individual profile fitting methods to the whole pattern. This can be accomplished with or without constraints on the cell, the space group, the structure, the profile shapes, their width, the h=f*g convolution$\ldots$ Is included in the review the silica glass structure modeling [3] by the Rietveld method [4]. This WPPF involves 22370 reflections overlapping so much that profile shapes do not really matter. Concerning less disordered homogeneous samples, it is shown that isotropic size-microstrain effects are accounted for by the standard full width at half maximum angular variation law. Anisotropic broadening effects may be approached if there is no sharp variation of shapes and widths as a function of hkl. Simple cases of faulting are treated by the separation of the associated broadened reflections from the others. Cases resisting to current WPPF approaches are emphasized, possible improvements are suggested.
On a theoretical point of view, it is stated that old size-% microstrain formulae are still valid in the application domain for which they were defined [5]. Crude approximations of these formulae leading to the description of broadened profile shapes by flexible analytical functions are shown to allow at least the idealized atomic structure refinement and even the *Fobs* extraction prior to the structure knowledge. It is concluded that these objectives are sufficient to justify a profile-shape approach two-thirds empirical, whereas inaccurate size-% microstrain information represents a non-negligible by-product. Possibilities of simulation/matching versus fitting the whole pattern are examined considering our current knowledge of the atomic structure of defects in complex compounds. Finally, analysis of imperfections along the WPPF route is not regarded as a " dead-end street" but as an inevitable challenge for the complete characterization of real materials, including those having hitherto an unknown structure.
1. DELHEZ, R. & MITTEMEIJER, E. J. (1994). Preface to
European Powder Diffraction EPDIC 3, Materials Science
Forum 166-169.
2. LE BAIL, A. (1992). Accuracy in Powder Diffraction II,
NIST Special Publication 846, 142-153.
3. LE BAIL, A. (1995). J. Non-Cryst. Solids 183, 39-42.
4. RIETVELD, H.M. (1969). J. Appl. Cryst. 2, 65-71.
5. WARREN, B.E. & AVERBACH, B.L. (1950). J. Appl.
Phys. 21, 595-599.