Solving with Superstructure in TOPAS using Rietveld refinement

originlab1

Hi everyone!
I would like to know if there is a way of Rietveld refinement in TOPAS for a superstructure. The situation is as below:
The sample being investigated has a monoclinic phase (space group is known to us). Also, the space group for the superstructure is known to us (it is hexagonal). Right now, I am performing the refinement by using two structures (one smaller monoclinic; second is hexagonal for the larger superstructure) which is more like performing refinement for a mixed phase. I would like to use a single structure where somehow I could accommodate the smaller monoclinic phase inside the bigger hexagonal superstrcture and then perform the refinement. Is that possible, both physically and mathematically? If yes, how do I accomplish that in TOPAS?

Please let me know. Thanks in advance,
--
AB

johnsoevans

Perhaps I don't understand your question properly. If the data is of a single phase sample in the superstructure form, then you only need the superstructure model in your Rietveld refinement. There's lots of ways of deriving superstructure models. One possibility is using isodistort. There are some examples on the Durham tutorial pages.

originlab1

Hi John,
Thank you for the rpely.
Are you referring to this: http://community.dur.ac.uk/john.evans/topas_workshop/tutorial_isoriet.htm?

The data may or may not be from a single phase, that is one more thing I am struggling with at this point.
Will keep you all posted.
--
Thanks,
AB

johnsoevans

Yes, that tutorial and/or some of the easier parts of the one on WO3.

If you do have a 2 phase system (whether this is likely depends on the relationship between the two phases - it's only likely if it's a first order phase transition) you'll need to be especially careful as the diffraction patterns from the two materials will be very similar. It would be easy, for example, for the superstructure to refine towards the higher symmetry phase and fit peaks from that phase.

originlab1

Got it..Thanks John :-)