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I have one problem regarding assining of atom using single crystal X-ray diffraction

I am working on MOFs

In one structure there are two metals Ni & Co

In asyymetric unit there are three metals

but I am not able to assign which is Ni & which is Co

What radiation did you use for the data collection?

If you use Cu radiation you will be right between the absorption edges of Ni and Co (enhanced element contrast), so
a good data collection (High Multiplicity of Observations) should give the answer to what element
goes on what site.

prince wrote:but I am not able to assign which is Ni & which is Co

This is intrinsically characteristic for X-ray diffraction.

Jeppe's idea might be helpful, but a neutron diffraction experiment would be the right choice. This is expensive and requires relatively large crystals however.

Why do you believe that the structure contains both Ni and Co? Do you have a good chemical analysis of the crystals (AAS or EDX)?

Thank You Jchrist and Rudi

From EDX I confirmed that crystal contains both Ni and Co.

I used Mo radiation for data collection.

It is not possible for me to go for neutron diffraction.

Should I collect data again using Cu radiation ?

What datacollection stratagy I should Plan?

Yes, try to recollect with Cu radiation.
For the strategy remember that Friedel pairs are no longer equivalent!
The difference between them is what carries the information about Ni an Co occupancies.
The best thing is if you can collect a full sphere data set, so you have all the Friedel pairs at leas once.
If you have time (and the crystal can survive the beam) a redundant full sphere would be better.

Your EDX result should also tell you what ratio of Ni:Co you have in your sample, this could help you refinement.

prince wrote: From EDX I confirmed that crystal contains both Ni and Co.

Ok. Are all three crystallographically independent metal sites chemically equivalent (e. g. same coordination numbers)?

jchrist wrote:For the strategy remember that Friedel pairs are no longer equivalent!
The difference between them is what carries the information about Ni an Co occupancies.

out of curiosity, why exactly are the Friedel pairs no longer equivalent in a centro-symmetric space group?

Marc

There has been no mentioning of the space group yet, so we don't know it it is centro-symmetric or not....
But you are right, in theory there is no difference on the Friedel pairs in centro-symmetric space groups.
Anyway the anomalous signal is still present and caries the desired information...

Thank you all for suggestions

The space group is P21/c

All three crystallographically independent metal sites having same co-ordination no. ie. 6 (octahedral)