Dear Tuong, From mathematical point of view it is obvious: the refinement yields the physically meaningless values for the components of ADP tensor:
the eigenvalues (=lengths of thermal ellipsoid axes) from your values are: -.1079639591e-2, .2901526405e-2, .1633111319e-1 What could be the reason(s) for that, depends on the data quality, structure model and many other things. From my experience, most often this happens when your refinement is overparametrized: you simply refine too many parameters = have unfavorable ratio of parameters-to-observables. Consequently, you may get somewhat arbitrary values of refined parameters, including ADPs, and they could easily become negative. Normally, if you have many atoms / complex structure and usual (laboratory diffractometer) data, you should not refine anisotropic ADPs for all atoms. In addition to that, in your case, once you'd mentioned a high value of refined isotropic ADPs (2.5), this could indicate some inappropriateness of the model: e.g. occupancy / occupancy distribution, which is strongly correlated with ADPs. Sincerely, Maxim. _________________ Maxim V. Lobanov Department of Chemistry Moscow State University loba...@icr.chem.msu.ru Friday, December 19, 2014, 7:28:16 PM, you wrote: > Dear all, > I'm new to Rietveld. I work with Powder X-ray diffraction and Fullprof. > Recently I've done a Rietveld refinement for a compound with the > space group P42/nmc (n°137) and during the process I saw a quite > high value of Biso of three atoms (~2.5). Then I had to refine > anisotropic temperature factors of these atomic positions. As a > result, two positions were well fitted, but the last one (the > general position) was not. I receive the message"Matrix U > non-positive definite!" in the .OUT file and then DIAMOND software > showed also an error on this position. > I would like to figure out the reason of this error. Can someone help me, > please? > Thank you a lot. > P/S: The values of the anisotropic temperature factors in Ang^2 > B11 = 0.01562 > B22 = 0.002507 > B33 = 0.000026 > B12 = 0.002869 > B13 = -0.001662 > B23 = 0.00142 > Tuong-Dan VU > Doctorante > IMMM (Institut des Materiaux et Molecules du Mans) UMR-6283 > Universite du Maine > avenue Olivier Messiaen > 72085 Le Mans Cedex 8 > France > Tel: 06 52 72 71 30 > e-mail: tuong_dan.vu....@univ-lemans.fr > vutuong...@gmail.com > >
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