As mentioned recently,
Their is no physical meaning to have negative thermal agitation factor.
It means that at high angle the observed data at upon the calculated.
It means that you should put more electron (if you are doing X-Ray
diffraction) or more electron or nuclear density in your
crystallographic model.
One of the usual problem could be:
- You have used some automatic variable slit !! and you try to refined
such data without correction.
- Check your crystallographic model
But if you have done such last things it should come from some
absorption problem : if you are using X-ray with high absorption
compound in reflection geometry this problem come quite often.
For example the simple crystallographic compound metal Platinum simple
cubic Fm-3m (N°225) Pt in 0,0,0
The absorption is big for copper radiation 4300cm-1.
I find -0.6ang2 with X'Pert HighScore Plus.
Normally we can make some correction for such absorption see :
/Référence/
/*20.- W. Pitschke, N. Mattern and H. Hermann, Powder Diffraction 8(4),*/
/* 223-228 (1993).*/
/ (In this paper the authors refine simultaneously all occupation/
/ numbers. This practice should be avoided. If the calculation were/
/ exact that gives rise to a singular matrix when the scale factor/
/ is also refined)/
/ See also/
/* W. Pitschke, H. Hermann and N. Mattern, Powder Diffraction 8(2),*/
/* **74-83 (1993).*/
/ /
/ Dans HighScore on trouve la reference :/
//H. HERMAN & M. ERMRICH///, Microabsorption of X-ray intensity in
randomly packed powder specimens, Acta Cryst. (1987), *A43*, 401 -- 405./
But I did not find so easy to apply such correction at least with
HighScore or Fullprof.
The data of the platinum or the molybdenum could be found in :
http://perso.univ-lemans.fr/~fgouten/RX_data
Best Whishes
François Goutenoire
Le 03/03/2010 17:08, carolina.zip...@fi.isc.cnr.it a écrit :
Dear all,
could someone explain to me the meaning of obtaining a negative Uiso
in GSAS?
I thought it was always positive...(p. 123 manual)
thanks
Carolina
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
Dr. Carolina Ziparo
Istituto dei Sistemi Complessi - sezione di Firenze,
C.N.R. - Consiglio Nazionale delle Ricerche
via Madonna del Piano, 10
I-50019 Sesto Fiorentino Italy
tel.: +39 055 5226693
fax: +39 055 5226683
e-mail: carolina.zip...@fi.isc.cnr.it
--
Dr. François Goutenoire
Laboratoire des Oxydes et Fluorures UMR6010
tel: 02.43.83.33.53
