> So I cannot let say that "Significantly different "physical" > size distributions could describe equally well the peak profile". > This is confusing. You may say that : significantly different > crystallite shapes could describe equally well the peak profile > in cubic symmetry. I am not sure that this sentence is > valuable equally for other symmetries when looking at all
Sorry, it seems me that rather your sentence is confusing, not mine. In the example with CeO2 the crystallites are quite spherical (one shape) even seen by microscope. But two significantly different distributions of the sphere radius (6a1, 6a2) (lognormal & gamma, respectively) given quite the same column length distribution (6b1, 6b2) and practically the same peak profile. It is no matter here of different crystallite shapes because the shape is unique (sphere). And also the cubic symmetry has no relevance, this should happen for any symmetry (I mean not an unique solution for the sphere radius distribution). (By the way, the sample of CeO2 in discussion is just the sample used in the round-robin paper that you co-authored; in this last paper we used only the lognormal distribution, but doesn't mean that this is the unique solution from powder diffraction). Concerning the different crystallite shapes, this is another storry. I said that even if the cristallites are not spherical, it is not obligatory to observe an anisotropic size broadening effect. Not spherical crystallites is only the necessary condition for size anisotropy effect, but not sufficient. The anisotropic size broadening effect is observable only if the non spherical shape is preferentially orientated with respect to the crystal axes (don't confuse with the texture). It is the case of your nickel hydroxyde in which the plate-like normal is preferentially oriented along the hexagonal c axis. But, if the not spherical crystallite shapes are randomly oriented with respect to the crystal axes (which is possible) the size broadening effect is isotropic and, only from powder diffraction, we can conclude erroneously that the crystallites are spherical. On the other hand, if the anisotropy is observed, the crystallite shape (and the distributions of specific radii) can not be uniquely determined only from powder diffraction. What we can determine is an apparent shape (and column lengths averages). Has any sense, in this case, to search for so called "physical models", or we have to be content with "phenomenological" findings (so much blamed, at least implicitely)? It is only a question, valid also for the strain effect. > So, let us have more fun with a size strain round robin on some > complex sample (or even a size-only round robin not on a > cubic compound ;-). I agree entirely. Best wishes, Nicolae Popa
