[Ifeffit] Sodium uranate fit problem

[Andy Connelly]

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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