The asymmetric ratio that is (R/O) should ideally be same for a series of varying concentration of Eu3+ ions. I have found papers where asymmetric ratio varies for different excitation wavelengths.
However, in my case the asymmetric ratio recorded at a particular excitation is varying significantly for different concentration of Eu3+ ions.
The asymmetric ratio is 2.06, 2.09, 2.15 and .721 at 1, 2, 3 and 5 mol % of europium. Why can this effect arise? Can anyone help?
The asymmetry ratio can very well change with Eu concentration. You substitute one ion with another and that can influence the site symmetry. A usual trend is that asymmetry increases with increasing dopant concentration as that disturbs the lattice. Once a threshold concentration is reached the symmetry begins to increase again as the crystal settles towards the Eu3+-endmember.
Maybe you can increase to dopant concentration up to 100% and see if there's a trend? On which site are you doping the Eu3+? If there is no trend, but the values just fluctuate, it might be your samples are of varying quality or there is a problem with the measurement.
I have some discussion regarding this matter in Aust J Chem 68(11) 1727-1734, full text available through my profile.
In adition to Florian's wise remarks: Your ratio is constant up to 3% (within experimental errors about +- 5%). Something is happening between 3 and 5%. You should investigate this more closely but running samples with 3.5, 4, 4.5% and correlating the data with XRD patterns.
Thank you all for your answers.
@Jean-Claude G Bünzli I also have the same thought process of making samples at 4 mol % and increasing in successive steps of .0.5 mol %. Further, my samples are glassy in nature they are not crystalline.
@Roman Kubrin I am talking about asymmetric ratio (5D0→7F2 /5D0→7F1).
@Florian Baur Thank you for the answer, and I will go through the mentioned literature. what do you mean by threshold condition here, can you please elaborate?
Regarding asymmetric ratio and other concepts that are often misused in europium spectroscopy, please see the article by Peter A. Tanner in Chem. Soc. Rev. 2013, 42, 5090-5101
http://pubs.rsc.org/en/content/articlepdf/2013/cs/c3cs60033e
Asymmetry ratios also depend whether europium ions are in the surface (where Eu symmetry is influenced/distorted by capping ligands) or in the interior of the particles.... if we are taking about nanoparticles here. So when we increase the Eu conc. and if more Eu ions replace surface lattice sites of the particles (which is expected) then their emission contribution will dominate and would increase the asymmetry ratio. At lower conc. the distribution of Eu between surface and interior would be more or less same and R value would be less as compared to higher Eu conc.
Further, I agree with JCB, that it should be ensured that increasing conc. is not changing the crystal structure of the particles, which should be monitored by XRD.
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