I am not sure if it is relevant for your case but in my work the reduction of particle size by grinding is important to get adequate accuracy.
See the work "P. S. Whitfield, L. D. Mitchell, The effects of particle statistics on Rietveld analysis of cement Zeitschrift für Kristallographie Supplement (2009) 53-59." On 22 March 2014 01:36, Frank Girgsdies <girgs...@fhi-berlin.mpg.de> wrote: > 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 >> > > > -- Laurie Aldridge 24 Balmer Cres Woonona 2517 NSW Australia Email laurie.aldri...@gmail.com Phone +61 (2) 4284 0132 Mobile 0403 718 522
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