What difference do you expect? For what I know, a fast electron hopping is present in the fe3O4. At 300K , the hopping produces a completely averaged spectrum from fe+3 and fe+2 ions which does not show a quadrupole effect. ( see. pages 251-254 of the Greenwood and Gibb book ).

@ Gabriele

As in the bulk we do get distinct spectra from tetrahedral and octahedral but when the particle size goes down to superparatism regime the sextets collapse to a single doublet. Why dont we see two doublet, one from A site and another from B site?

Pure tetrahedral and octahedral environments give rise to a null ligand contribution to the EFG tensor. Moreover the electron hopping tends to average isomer shifts and EFG valence contributions.

Electron hopping is considered only among the Fe+2/Fe+3 ions residing at octahedral. Would you like to conclde that in superparamagnetic Fe3O4, electron hopping occurrs among Fe in tetehedral and octahedral sites.

@ Herojit

I did not see your data and moreover I'm not working with nanoparticles ... but:

What differences do you expect between the corresponding parameters for the A and B sites?

What is the line width value?

Did you consider nonhomogeneous line enlargements?

Are you sure that sextets fully collapsed?

The value for the sample thickness ( or at least for the spectrum area) is it in agreement with the quantity of material you used ?

As reported in literature particle size when less than 10 nm give a doublet. The confusion with me is why do all the superparamagnetic(SPM) iron oxide and hydroxide gives almost similar doublet. Even though Hematite, magnetite and goethite have different structure and magnetic properties? What is the physics behind this similar results? ZnFe2O4 have IS=0.3 mm/s, QS = 0.34 mm/s but when goes to superparamgnetic regime QS increases to about 0.6 mm/s almost similar to SPM Iron oxide. The reason for the increament was reported to be due to defect in nano particles but coincidently matches with SPM Iron oxide.

Whether it is all dominated by the two spin state of Superparamagnetism.

@ Gabriele

Yes, the sextets of Fe3O4 got collapse to a single sextet as particle size is approx 10 nm and then finally ended up with a doublet when size goes down to 10 nm. The A sites and B sites are not distinguishable as in the bulk case.

I am not expert of Mossbauer Spectroscopy but I just want to add comment for the asked thread. Mossbauer measuring relaxation time is even better than XRD for the specific element say Fe. Lab XRD cant detect minor segregated Fe phase which Mossbauer can. Similarly, when particle size reduced, i.e. around 10nm (my experience is even higher nano size depending upon spinel compound) at room temperature. Mossbauer gave doublet for say nanoparticles of ZnFe2O4, NiFe2O4 and so on. This doublet is due to fast spin relaxations between octa and tetra sites whch is beyond the limit of Mossbauer relaxation time. Mossbauer than cant differentiate between the interactions of A and B sites interaction and gave overall doublet pattern which is defined as superparamagnetic structure.

@ M Nadeem

I agree that due to fast relaxation time it arises a doublet else it would have been sextet. But still the chemical environment is different as of A and B site. How does the Isomer shift of the A site and B site comes out to be same?

@ Herojit

Defects increase Qs values.

A second source of the increase is related to the direction of the magnetic hyperfine field. You know that the measured value for Qs is underestimate if distinct quantization axes characterize the quadrupolar and magnetic hyperfine interactions; consequently the collapse of the magnetic interaction gives rise to an increase of the measured value for the QS.

Finally, you noted that your spectra show a two step evolution from a couple of sextets to a single doublet.

This suggests a quite complex dynamical process and complex dynamical processes may also give rise to "ghost lines" ( not corresponding to static and mean level positions). In particular give a look at the paper " J. Phys.: Condens. Matter 20 (2008) 505201". It predicts "... the appearance of 57Fe magnetic sextets .... which look like effective ‘doublets’ of lines often observed in experimental spectra [ of nanoparticles ]."

A superparamagnetic crystallite may be considered as a particle with spin S.

Typically, S is of the order of 104. Such a particle maybe found in one of

2 S + 1 =2 X 104 states.

For calculation of the M S, transition sbetween any two states have to be

considered.This involves the inversion of a matrix of the order of

2 X 104. However, as the transition rate is high for transitions among states

which are not separated by anenergy barrier (states close to the same energy minimum)

we may simplify the calculations considerably by limiting ourselves to consider only

two states and the transitions between them, namely the thermally averaged states

to always obtain similar MS

A superparamagnetic crystallite may be considered as a particle with spin S.

Typically, S is of the order of 104. Such a particle maybe found in one of

2 S + 1 =2 X 104 states.

For calculation of the M S, transition sbetween any two states have to be

considered.This involves the inversion of a matrix of the order of

2 X 104. However, as the transition rate is high for transitions among states

which are not separated by anenergy barrier (states close to the same energy minimum)

we may simplify the calculations considerably by limiting ourselves to consider only

two states and the transitions between them, namely the thermally averaged states

to always obtain similar MS

A superparamagnetic crystallite may be considered as a particle with spin S.

Typically, S is of the order of 104. Such a particle maybe found in one of

2 S + 1 =2 X 104 states.

For calculation of the M S, transition sbetween any two states have to be

considered.This involves the inversion of a matrix of the order of

2 X 104. However, as the transition rate is high for transitions among states

which are not separated by anenergy barrier (states close to the same energy minimum)

we may simplify the calculations considerably by limiting ourselves to consider only

two states and the transitions between them, namely the thermally averaged states

to always obtain similar MS.