> Alan and Maxim, Thanks for the comment and the article. I relieved that I know the point.
> Leonid, Yes, the instrumental resolution itself increases with d (or TOF). But it is still strange for me that only all-odd peaks show different d-dependence from CeO2 and other all-even peaks in terms of slope in the delta-d/d vs d plot. Now, I think a similar situation as high temperature phase of Mg(BH4)2 occurs in my quaternary Heusler sample. For all-odd hkl, structure factor is F_hkl=4(f_A-f_C)+/-4i(f_B-f_D). Here, A-D denote four fcc sublattices in Heusler alloys, or 4a,4c,4b,4d sites in F-43m. If there exist ABCD and CDAB type domains, those domain have out-of-phase scattering for all-odd reflections and same story as Mg(BH4)2 can be applied. But still I don’t understand why peak widths show such strong dependence on d (or TOF). Concerning attachment files. This time I use Dropbox but I don’t guarantee it as an image archive because the image might be removed by me a few years later when I clean up my folders. //================//================// Kotaro SAITO High Energy Accelerator Research Organization Institute of Materials Structure Science 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan //================//================// > 2015/08/04 19:34、Alan Hewat <alan.he...@neutronoptics.com> のメール： > > On 4 August 2015 at 11:54, Kotaro SAITO <kotaro.sa...@kek.jp> wrote: > Or do I miss some basic points about diffraction? > > I won't try to address your specific material... and I'm being called to > lunch :-) But for beginners who may be lost in these technical papers, I will > attempt the following trivial explanation > > If you have a layered material where two layers A and B are slightly > different you will have super-structure reflections. These will be as sharp > as the main reflections (from the average structure) if the order of the > layers is perfectly regular ABABABAB... > > But if the layers only have short-range order eg ABABBABAAB... then these > superlattice reflections will be broadened, and even completely washed out if > the order between layers is completely random. Otherwise the width delta-d of > the superstructure reflections will give you the short range order length - > the shorter the correlation length the broader the superlattice reflections. > > Obviously delta-d doesn't depend on the d-spacing between layers, only on the > length of their order. So the broadening is constant in d-space as usually > plotted for TOF neutron diffraction. > > For angular dispersion eg with a constant x-ray or neutron wavelength, > Bragg's law 2d.sin(theta)=lambda comes in. If you differentiate Bragg's law > you will find a simple relation between delta-d and delta-2theta, the line > broadening for angular dispersion measurements. > > Alan. > (Everything should be as simple as possible... but no simpler.) > BTW, thanks for using dropbox instead of an attachment. That's the way to > go... > -- > ______________________________________________ > Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE > <alan.he...@neutronoptics.com> +33.476.98.41.68 > http://www.NeutronOptics.com/hewat > ______________________________________________ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com> > Send commands to <lists...@ill.fr> eg: HELP as the subject with no body text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >
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