Hi Norberto, just my two cents: If you observe a discrepancy between observed and calculated intensities, then it could also be that the calculated intensities are wrong. Depending on the software you use for the Rietveld refinement, it may happen that the wrong combination of space group setting and atomic coordinates is used. Fd-3m has two alternative origin choices, which would yield quite different calculated intensities when combined with the same set of coordinates. At least in Topas, which I'm using, this is one of the two most common sources of error.
Best wishes, Frank On 21.03.2014 10:59, Norberto Masciocchi wrote:
Dear Friends, I am facing a problem in quantifying magnetite in a complex natural mixture, using the conventional Rietveld method (in its quantitative analysis approach). Apparently, the octahedral morphology of (difficult to grind) magnetite crystals affects itd diffraction pattern. To my knowledge, as certified in the original paper (JAC 1986, 267-272), the ubiquitous and highly performing March-Dollase equation, holding for "any" crystal symmetry, only applies to inequant crystallites (i.e. crystallites with unequal sides), or, better said, to "effectively rod- or disk-shaped specimens" (provided that the geometry of the experiments keeps cylindrical symmetry). Neither octahedral nor cubic (e.g., NaCl) crystals can be considered "inequant". So, the question is: Is there any way do get around this problem (without resorting to spherical hamonics or to grind the specimen in a WC, SiC or BN mill)? Thank you for your patience. Norberto
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 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/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
