Re: Domain Size confusion
I also strongly support Armel Le Bail viewpoint about the coherent diffracting domains or coherently scattering regions. This is what is measured with this method. Coherently scattering regions maybe much smaller (sometimes several orders of magnitude) than the grain size. Unfortunately there is a lot of confusion in the literature and many respected authors still claim that they measure the grain size from line broadening while they really measure the coherent domain size (which might differ several orders of magnitude). I think that this discussion is very important. Rosa Barabash -Original Message- From: Armel Le Bail [mailto:[EMAIL PROTECTED] Sent: Wednesday, November 17, 2004 2:41 PM To: rietveld_l@ill.fr I will be grateful to all if there is a discussion on the term Domain Size over this list. Domain size is an incomplete definition. Line broadening due to size effect is related to the definition of coherent diffracting domains. The coherency comes from the fact that the three-dimensional ordering is not broken inside of these domains. That three-dimensional ordering may show small deviations and in such a case, these deviations will induce another effect : broadening due to microstrain. Equations on size effect take account of columns of cells perpendicular to the direction of diffraction. Where these columns are interrupted are the limits of the coherently diffracting domains. If the presence of a dislocation may not always completely break the three-dimensional ordering, at least it will induce strong local perturbations leading to microstrain effects. As you can see, putting a clear limit of coherently diffracting domain size is not that simple in the presence of defaults... All this finishes in a statistical description. The structure may be so distorted that one cannot decide for a clear place for locating where a column of cell is interrupted... Some twin boundaries will break the three-dimensional periodicity and delimit clear domains. The domain size may well concern a subset of the cell content, for instance in a perovskite-type structure MX3, the M framework can be quasi perfect in spite of domain boundaries induced by inversion of tillting angles of the MX6 octahedra. Some lines may be narrow and other will be broad (antiphase domains). Etc - At least there is no conceptual problem to imagine larger grains formed by a conglomerate of disoriented coherently diffracting domains, glued with some cement at the boundaries. The cement being frequently highly distorted, if not completely amorphous. In distorted materials, some guys place the average limit of the coherently diffracting domains exactly where the three-dimensional order vanishes statistically due to disorder increase at a certain distance from an origin. In such compounds, no clear domain size limit would be observed. Still confusing... Armel
partial occupancy
Hello Doinita, It might be that you have some order in the occupancies of V, O and vacancies. Did you check for this possibility? Rosa Barabash From: Doinita E Neiner [mailto:[EMAIL PROTECTED] Sent: Friday, November 05, 2004 5:53 PM To: [EMAIL PROTECTED] Subject: partial occupancy Dear list, I am working with a tetragonal cell that has a vanadyl unit.I think that both of the atoms, Vand O, arein thesame special position. The problem is that both have partial occupancies on the site, .5 and .5, and GSAS does not handle them very good (or I am doing something wrong). It keeps on putting them to form very unlikely bonds with each other. Knowing that the participants at this list are very knowledgeable in the matter, I would appreciate very muchany kind of suggestions. Thank you in advance, Doinita
anisotropic line broadening
Hi, One should be careful with interpretation of anisotropic line broadening. Anisotropic line broadening can be due to the formation of small dislocation loops, dipoles or some early stages of pre-precipitation (GP zones) in the alloy. Would be nice if you can obtain information about diffraction intensity in rocking direction. Thus you can verify the parameters of dislocation structure you obtained. See, for example, R.I.Barabash, P.Klimanek, Phenomenological and microscopical description of scattering by different dislocation arrangements, Zeitschrift fur Metallkunde, 1, 70-75, (2001) Research Prof. Rosa Barabash, Oak Ridge National Laboratory, Metals and Ceramics Div. One Bethel Valley Road, Oak Ridge TN 37830 Phone (865) 2417230 e-mail:[EMAIL PROTECTED]
