[Ifeffit] Sodium uranate fit problem

[Shelly Kelly]

Could be a split Oeq shell.

Shelly

On Fri, Jun 11, 2010 at 5:19 AM, Andy Connelly <andyjconnelly at gmail.com> wrote:
> Hello all,
>
> I've been following this mailing list for ages and have learnt a lot,
> thank you. Now I have a problem and am hoping someone can help.
>
> I have some really nice sodium uranate (Na2U2O7) data from the NSLS
> (X23A2) that I collected (embarrassingly) a couple of years ago. I
> have been trying to fit it and have never managed to get a
> satisfactory fit even to the first shell.
>
> >From the literature there should be 2 uranyl oxygens and 6 equatorial
> oxygens. If I fit these paths I get an amp (S02) value of 0.32 and
> negative sigma squared values (ss). I have tried to add paths from the
> second shell to reduce this amp value but no success. I do not believe
> that this value is real but I cannot find any problem in the
> background or in the fit. I have found similar problems for lithium
> uranate and some glass samples, all of which should have similar
> uranium coordinations. I can bring ss into a more sensible value but
> amp is always very low. I have tried several different .cif files for
> different uranate or uranyl systems and they all give roughly the same
> result.
>
> I have found similar problems with some data I collected at Daresbury
> on a different Na2U2O7 sample so I do not think it is the sample or
> the beamline. Does anyone have any thoughts why this might be so?
>
> Any help or suggestions would be most appreciated.
>
> Thank you all in advance,
>
> Best regards
>
> Andy Connelly
>
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