Re: [sdpd] Re: UPPW-5 solution - UPPW-6 problem

[Jon Wright <wright...@esrf.fr>]

Alan Coelho wrote:

>UPPW-5 is a case where powder data does not yield a unique solution. 
>When multiple solutions yield similar "perfect" Pawley/Le Bail fits with 
>similar de Wolff  values then it is not a matter of failure of the 
>programs/methods but rather a failure of the data to yield a unique 
>solution. In my view it is therefore not possible for any indexing 
>method to resolve the ambiguity.
>
Seems like the challenge is to get those unit cells where the difference 
between the "true" solution and the many false solutions is difficult to 
distinguish, even if the correct cell appears in the list of possible 
solutions. Is it possible (with Topas or any other programs where 
authors might be listening) to exploit any "extra" data which can help 
to select the "true" solution. One might like to multiple datasets 
collected under different conditions, to exploit things like anisotropic 
line shifts (temperature/pressure/radiation damage), changes in 
intensity due to texture, or even changes in line width due to extra 
grinding of annealing. Since these things are hkl dependent, usually in 
some understandable way, it should help to eliminate some of the 
candidate cells. Currently it seems like a lot of "manual intervention" 
would be needed to analyse any data like that.

Given a real sample where you cannot find the unit cell, it does not 
seem too arduous to collect more than one powder pattern, but it still 
seems very difficult to do anything with the extra data if it is 
collected. Can Topas/SVD index help here; is it just a question of 
knocking up a clever .inp file? The least squares method seems like it 
can be extended to fit hkl dependent functions for proposed cells more 
easily than some of the other algorithms. Perhaps a lot of work, but 
potentially useful if the unit cells produced were more reliable....

Regards,

Jon


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