rietveld of 2 phases

fatma

Hey everyone! I am doing multiphase rietveld refinment of my data. My material has two phases alpha and beta.In alpha phase Ni 1 and Ni 2 are octahedral and Mo1 and Mo2 is also octahedral but in beta phase Ni 1 and Ni2 are octahedral and Mo1 and Mo2 are tetrahedral.
i am just not understanding what to fix and what to refine regarding atomic positions. can any one suggest me which atoms to fix or which to refine?
in alpha phase Ni1 is attached to Oxygen at position 1,2,4,5
Ni2 is attached to Oxygen at position 1, 3 and 4
Mo1 is attached to oxygen at position 1, 3 and 4
Mo2 is attached to oxygen at position 1 , 2 and 5
In beta phase Ni1 is attached to Oxygen at 1,3,4,5
Ni2 is attached to 1,2 and 4
Mo1 is attached to Oxygen at 2,4 and 5
Mo2 is attached to oxygen at 1 and 3.
Kindly suggest any combinations i am badly stuck on it
Geometry of both phases are monoclinic

johnsoevans

This is more of a crystallography question than a topas question. Which atomic coordinates you can refine is determined by the crystallographic sites the atoms occupy, not by their coordination environment. You can only refine xyz parameters which are free to refine by symmetry. Whether it is sensible to refine those parameters depends on the data quality and the atoms involved. e.g. in the X-ray powder pattern of a bismuth oxide the bismuth will dominate scattering to such an extent that the data may not be sensitive to the oxygen coordinates.

This type of question should be covered in most crystallography texts (e.g. Bill Clegg's Oxford Primer on crystallography is one good place to start). Or you could ask a local crystallographer (powder or single crystal) to go through things with you. Hope that helps.

fatma

Thanks alot