Dear Colleague, I am somewhat confused to this discussion. As Bob said, one can put all Co in one site & all Fe in the other with the duplicate sites set to zero FRAC. Then put in constraint that forces amount of Fe lost from A-site to appear in the B-site and vise versa for the Co, which can constrain the ratio of the Co:Fe to be 1:2 (allowing the constraint factor). But how to keep the full occupancy of each atom site, for example site A or site B? Could you like to give me one constraints to suffice the above two conditions(the ratio 2:1 and the full occupancy of each atom site)? I am sure that this constrain can be acieved, but I can't. Would you like to tell me the detail?
In other word, we may introduce the atoms as follows: site A: Fe1=0, Co1=1 site B: Fe2=1, Co2=0 one can put all Co in A site & all Fe in B site with the duplicate sites set to zero FRAC.Then , the constraints should be introduced as follows, where d represents the shifts on occupancy parameters(allowing for the constraint factor): dFe1 = -dCo1 = -dFe2 = dCo2 so that the Co atom lost from site A is compensated by Fe atom in the same site and is transfered to site B, and vice verse for Fe atom, which can keep the ration 2:1 and full occupancy of each site. However, Due to the way GSAS implements constraints, a parameter can be included in only one constraint. So I don't know how to do? Any advice would be greatly appreciated, Thanks a million in advance, Best regards, Jianrong Chen School of Arts and Science University of Louisville > > > Re: Chemical Structure Constraint > > -------------------------------------------------------------------------------- > > From: Darin Hoffman > Subject: Re: Chemical Structure Constraint > Date: Tue, 10 Aug 2004 07:21:44 -0700 > > -------------------------------------------------------------------------------- > Thanks Bob. I think I understand how to do that. I will give it a try today. > > > -Darin > > At 07:26 AM 8/10/2004, you wrote: > Dear Darin & others, > The trick here is to realize the effect of the different site > multiplicities. Moreover, remember that the constraint applies to the > shifts & not the values. So start the site fractions with the right > stereochemistry - you can put all Co in one site & all Fe in the other > with the duplicate sites set to zero FRAC. Then put in constraint that > forces amount of Fe lost from A-site to appear in the B-site and vise > versa for the Co; a single constraint with 4 terms should suffice. The > constraint factors will be 1.0 or 0.5 to account for the site > multiplicity changes. All this assumes no vacancies - you can't really > determine that from a single experiment, too many variables & not > enough information. > Bob Von Dreele > > > ________________________________ > > > From: Darin Hoffman [mailto:[EMAIL PROTECTED] > Sent: Mon 8/9/2004 5:23 PM > To: [EMAIL PROTECTED] > > > > > > Hello all: > > > I am currently working on a rietveld refinement of CoFe2O4 using GSAS. The > structural model I am using is > Site A 1/8, 1/8, 1/8 > Site B 1/2, 1/2, 1/2 > Oxygen site 1/4+d,1/4+d,1/4+d > > > On site A and B both Iron and Cobalt are present. > > > I am trying to create a constraint that forces the ratio of the Co:Fe to be > 1:2 as well as force the fractional occupancies of each site to inverse > each other . In other words percent Iron goes up percent Cobalt goes > down. This part I have been able to do quite easily. I thought I was able > to set up the 1:2 ratio constraint correctly but I don't believe the answer > because I am getting the Cobalt on both sites to have a Fraction value of > ~0.28 and the Iron fraction to = ~ 0.57. I know that I may have vacancies > but I don't believe both site should be the same. > > > Can anyone please tell me how to set up a chemical ratio constraint either > going through the EXPGUI or DOS interface? > > > Sincerely, > > > Darin Hoffman > > > ################################################# > Darin Hoffman, TSPA > Research Intern, NPDF > Lujan Neutron Scattering Center > Los Alamos National Laboratory > e-mail: [EMAIL PROTECTED] > PH: 505-667-8704 > #####################################################