Dear Keka, if you have a powder sample, I guess that the doublet you found is due to a superparamagnetic fraction of the particles, that are blocked at 4.2K (for the time window of Mossbauer measuring time, which is about 10^-8 s). Since they are blocked, these particles will contribute to the sextets, and no longer to a doublet. As a very rough rule, based only in my experience with superparamagnetic particles, almost all doublets coming from this situation have variable IS values, but the QS is close to 0.3 mm/s. Is that your case?

The changes in area are also, as you probably know, dependent on the recoiless free fraction and this can change with temperature more or less depending on the material.

I thing that is not obvious at all and I cannot explain is that, at 4.2 K, ALL particles (except very, very  small ones) should be blocked, i.e., no doublet should arise unless it is due to an actual paramagnetic phase. 

Hope this helps. 

Dear Prof G F Goya,

The particle size is in microns so how can there be superparamagnetism?

Keka

The samples have been synthesized by the ceramic tecnique. So how can there be nanoparticles. What is Ku.

Keka

I'm also long time away from Mössbauer. You may have particles of all sizes, since it is polycrystalline and I guess you haven't homogenized the sample to one size. There are effects derived from that (see the link below). I don't know your material so as to discard different domains within the particle. I used to relate the area of the spectral contribution to the concentration of a particular sample, but all at the same temperature, because of the change in the recoil fraction with temperature.  I fully agree with Goya.  All the best with your data!

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