Dear Sir. Concerning your issue about hw to differentiate between two Lanthanides in the same crystal structure. The similarity and difference between the solid state properties of the 4f and 5f transition metals are pointed out. The heavier 5f elements show properties which have direct correspondence to the early 4f transition metals, suggesting a localized behaviour of the 5f electrons for those metals. On the other hand, the fact that Pu metal has a 30% lower volume than its neighbour heavier element, Am, suggests a tremendous difference in the properties of the 5f electrons for this element relative to the heavier actinides. This change in behaviour between Pu and Am can be viewed as a Mott transition within the 5f shell as a function of the atomic number Z. On the metallic 5f side of the Mott transition (i.e., early actinides), the elements show most unusual crystal structures, the common feature being their low symmetry. An analogous behaviour for the lanthanides is found in cerium metal under compression, where structures typical for the light actinides have been observed experimentally. A generalized phase diagram for the actinides is shown to contain features comparable to the individual phase diagram of Ce metal. The crystal structure behaviour of the lanthanides and heavier actinides is determined by the number of 5d (or 6d) electrons in the metallic state, since for these elements the f electrons are localized and nonbonding. For the earlier actinide metals electronic structure calculations - where the 5f orbitals are treated as part of the valence bands - account very well for the observed ground state crystal structures. The distorted structures can be understood as Peierls distortions away from the symmetric bcc structure and originate from strongly bonding 5f electrons occupying relatively narrow 5f states. High pressure is an extremely useful experimental tool to demonstrate the interrelationship between the lanthanides and the actinides. I think the following below links and the attached files may help you in your analysis:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4165975/pdf/srep06398.pdf

http://repositorio.uchile.cl/bitstream/handle/2250/124508/Baggio%20R-Crystal.pdf?sequence=1

Thanks

Depending on the lanthanide ions you are working with there is the possibility to discriminate them by single crystal XRD data if you have a good crystal, but if the different lanthanide ions are very close (for example, Yb and Er) it can't be solved.

If the compound is a small molecule, you can obtain information by ESI-mass which will give you an exact picture of the type of molecular species in the sample. To deconvolve ESI-Mass spectra and obtain a quantification of the different possible (hetero- and homo-metallic) species you also need the precise amount of the different ions in the sample (possibly by ICP-MS or AES, XRF would also do). Take into account that, unless you had a very well-thought chemical design, you will have a sample constituted by different species as similar Ln ions mix randomly!

Luminescence IS definitely another (very accurate!) option, through the analysis of the temporal dynamics, but it can be tricky to interpret.

Take a look at these:

Article Ln 3 Q 9 as a Molecular Framework for Ion-Size-Driven Assemb...

Article Fully Efficient Direct Yb-to-Er Energy Transfer at Molecular...

Thank you so much, Dear Ma'am and Sir. Though it will take me a while to follow up all the good ideas you have introduced me to, I am very optimistic that some of them might help me a lot. I will keep this post updated.

Thanks again

Regards

Shailabh