Hi, > It seems that we disagree on the meaning of some > english words. English is not my mother language, so I may be > wrong.
Nor mine, so I can be equally wrong (or worse). > I was able to put one word on that definition (thanks for it) in my > previous email : distribution (a size distribution). > > In these earlier works (maybe you define any earlier work as > being "naive" ?) it is not at all the crystallite shape which is > approximated by an ellipsoid. The ellipsoid is there for > modelling the variation of the average size M(hkl) (which is > the mean of the size distribution). If ellipsoid models the crystallite shape is an approximation, good or not good, if models the average size "seen" in powder diffraction as function of direction is a mistake (see next comment). > > So, thanks, I used ellipsoids in 1983-87 for describing some > simple size and strain anisotropy effects in the Rietveld method. > I think that no elementary principle was violated, though Presume one of your students makes a fit on a sample having only size anisotropy and he is able to determine the six parameters of the ellipsoid. But after that he has a funny idea to repeat the fit changing (hkl) into equivalents (h'k'l'). He has a chance to obtain once again a good fit, with other ellipsoid parameters but with (approximately) the same average size, this time in other direction [lamda/cos(theta)/M(hkl)=lamda/cos(theta)/M(h'k'l')]. Am I wrong? If not, how you explain him that averages of the size distribution are the same in different directions once were approximated by an ellipsoid? And what set of ellipsoid parameters you advice to consider, the first or the second one? > particularly stupid. The ellipsoid method was applied to the > recent Size-Strain Round Robin CeO2 sample, giving > results not completely fool (in the sense that not a > lot of anisotropy was found for that cubic sample > showing almost size-effect only, and quasi-isotropy). Not surprisingly. Take a sphere and put two cones at the ends of one diameter. Certainly the finite high of cone means anisotropy. Refine the same CeO2 pattern. Very probably you will find zero for the cone high. Is this funny model equally good? > cubic case showing strong stacking fault effects for HNbO3 > (cubic symmetry). A neutron pattern is available. I would be interested > in a better estimation of the size and strain effects on that sample > (not only a phenomenological fit). Can you provide that better > estimation ? > > Best wishes with HNbO3, > > Armel Le Bail Strong staking faults effect? I would accept your challenge, but I'm not sure that with a knife in place of scissors is possible to do easy tailoring. That doesn't mean the knife is good for nothing. Best wishes and ... il faut pas s'enerver Nicolae Popa
