>
> >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?
>
> But why to presume so soon that people are dumb ?
>
> You may also presume that the student is not stupid enough
> for trying to determine the 6 parameters of the ellipsoid in any case
> and that he applies restrictions related to the symmetry, as
> recommended in the software manual (the software name was
> ARIT)... That manual says that the 6 parameters are obtainable
> only in triclinic symmetry, etc.
>
> I prefer to presume first that people are smart, and may be change
> my opinion later.
>
> I guess that the Lij in GSAS are explained to be symmetry-
> restricted as well.
>
> Armel
>
By contrary, I presumed a smart student observing immediately that by
applying to the ellipsoid the symmetry restrictions he obtains some strange
ellipsoids: for orthorhombic the principal axes are always along the crystal
axis, for trigonal, tetragonal & hexagonal they are always rotation
ellipsoids with 3,4,6 - fold axis as rotation axes. He could ask the master
how is the nature so perfect. How know the crystal to grows always along the
symmetry axis? But the most wondered will be the student
seeing that for cubic crystals the ellipsoid is in fact a sphere. To not
risk the next examination probably he will not put this question: how then
you
searched for size anisotropy in CeO2 with ARIT? Or the symmetry
restrictions are optional?
Nicolae Popa
