neutron diffraction

Sunil Kumar

Can you please Explain .. 

How ??

Use Raman spectroscopy or grazing incidence in-plane x-ray diffraction (GIXD)

I guess that the Mössbauer spectroscopy can help you.

You can do single crystal XRD refinement

I, humbly, submit four points about the change of Co^2 to Co^3 in a Co-ferrite.

[A]This phenomenon is ,many a times, noticed due to photon induced charge transfer reaction:

Co^2+Fe^3→Co^3+Fe^2. -----[1]

[B]In fact, it is also a cause of hoppong phenomenon.

[C] I agree with respected Mdm. Daniela Gogova

that this change can be studied by the Raman technique.

[D] Now two question remain unanswered :

[i] how to study the extent of change over of Co(II) to Co(III)?

[ii] which sites the newly formed Co^3 occuly?

No doubt, I will go for Mossbauer spectra but IN AN INDIRECT WAY as follws:

In normal MS of Co- ferrite, we SHOULD observe to sextets; the inner sextet arising from Fe(III)HS present in tetrahedral sites and the outer sextet arises from Fe(III)HS present in octahedral sites.

Their quanization can be studied from area / heights/ intensities of the peaks[ the method is reported in literature and even the questions have been answered by our RG friends].

As per eqation[1], when Co^2 changes to Co^3, there will be a corresponding change of Fe^3 to Fe^2. In away,decrease in concentration of Fe^3 will be equal to an increase in concentration of Co^3.

Now regarding the sites which these newly formed Co^3 would occupy:

Their ionic radii are = Co^2=0.74. ;Co^3= 0.63 ; Fe^2=0.76 and Fe^3=0.64 A. As Co^3 and Fe^3 have almost same radii, so Co^3 should prefer the same sites which were previously occupied by Fe^3.

We previously do it by EPR. It is an excellent technique to understand the oxidation state, symmetry, etc. Use EPR spectra to identify the the oxidation state of Co and then from analysing g and A values you can understand whether it is in tetrahedral or octrahedral

But if your sample is in powder form, it is rather difficult due to large no of interactions like NQR , ENDOR etc. If you are not a expert in EPR, then do it in the single crystal form. It will be very easy to detect and understand.

But sir, I have to submit the following points:

[I]We will not be able to detect Co^3[ Diamagnetic] by ESR.

[II] ESR is expected to give ESR peaks from both Co(II) and Fe(III).

[III] We may not able to distinguish these two ions from their ‘g’ and ‘A” values. Yes, we can make a distinction  between  tetrahedral  and octahedral  if there were present  Co(II)  alone.

Co3+ electronic configuration 1s2 2s2 2p6 3s2 3p6 3d6 . With four unpaired electrons in the d orbital, electron spin s=2 will spilt into 2s+1 levels and with selection rule +/-1, it will give spectra.  it is paramagnetic. The possible spin states for Co(III) complexes are bit controversy with either they can exist in intermediate spin state (s=1) or high spin state (s=2). Both gives EPR signals. Also in some complexes due spin cancellation Co(III) may exist as diamagnetic. But this can be easily routed by studying low temp EPR. Also for compounds we cannot expect all exist in 2+ or 3+ states. Some time overlapping of these states also exist. All these can be analysed beautifully by EPR.         

One way is the generate the experimental or theroreical electron density distribution for the material (perhaps more than you want to do) , evaluate the bond critical point properties for the atom and count the number of bond paths(the number of bonded interations that define the coordination number) that radiate from the atom. According  to Richard Bader, the coordination number is equal to the number of bond paths that radiate from the atom to its coordinating atoms.

For simple oxide systems we can predict whether a transition metal ion adopts a Td or Oh hole based on crystal field site preference energy. For spinel AB2O4 the 4 O2- ions are charge-balanced by an A2+ ion and two B3+ ions. Normally the higher charged B3+ ions adopt the Oh sites and the A2+ ion sits on the Td site. However there are exceptions when the A2+ ion has a crystal field stabilisation energy under Oh symmetry (and the B3+ ion doesn't). For example in NiFe2O4 the Ni2+ would normally go on the Td site and the Fe3+ on the Oh site. however Fe3+ (d5 configuration) has 0 crystal field stabilisation energy (0 CFSE) so doesn't care whether it is on an Oh or Td site. conversely the Ni2+ is d8 and under Oh symmetry would gain -6/5 Delta of CFSE. So the Ni2+ ion has a strong preference for the Oh site and so, in this case, the ions would swap over (known as an inverse spinel). See crystal field stabilisation energy here:

http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Crystal_Field_Theory/Introduction_to_Crystal_Field_Theory/Crystal_Field_Stabilization_Energy

Hope that helps. - you'll need to compare your Co3+ with the other ions present in your sample....

For Dr. Sridharan Ravi

Pls. treat it as an academic discussion. And in an academic discussion- no one loses; both win.

[A]I agree that the energy difference between the two spin states of Co(II) and Co(III) is only ~ 0.03 eV, and thus transition between the two states can be thermally driven, where at low temperatures Co(III) is in the LS state and at higher temperatures it is in the HS state.

{B} I also do not deny ± 1 rule .

(i)But  then CO(IIHS) MAY show a triplet[3/2---1/2; 1/2- (-1/2); -3/2---(-1/2)].

(ii) Are the Co(IIIHS)peaks and tripet of Co(IIHS) so well separated in ESR of Co-ferrites that they do not overlap with each other or Co(IIHS) triplet is not observed at all?

{C} Permit me use word” rogue”for Co(II) where it can also have an EFFECTIVE SPIN =1/2 .

{D} I still have an other point to clarify . This means that at a CERTAIN TEMPERATURE{ YOUR GOODSELF KNOW BETTER} when we scan the ESR of Co-ferrites , THEORETICALLY there are so many possibilities such as- the presence of Co(III HS), Co(II HS), Co(II; effective spin=1/2), Fe(III HS/LS), Fe(II HS). Do the Co- ferrites show only Co(IIIHS) at THAT CERTAIN DEFINITE TEMPERATURES while all other ESR transitions are not observed at all?

{E} Lastly, could you pls. send me one ESR spectrum among your own Co- ferrites depicting ESR lines of only Co(IIIHS) along with calculated :g” and “A” values?

There is no need to theoretically predict the answer. Measure it! As a couple others mentioned, either X-ray or Neutron diffraction will give you the definitive answer, as the diffraction peak intensities will be dependent on the position of the Co within the unit cell.

Provided the cation position is trivalent and cation radius is suitable the activation by rare-earth ions might help. RE f-f transitions are forbidden in the octahedron and resolved in  tetrahedron. Correspondingly, the absorption of these ions is weak in the first case and strong for the tetrahedron. For bivalent cation the same way may be applied with doping by transition-metal ions. It is not a panacea but useful technique.

I agree with Lawrence Margulies.

@ Manohar Sehgal. Thank you for your detailed descriptions. At present we are not working with EPR, but  I still feel it a great tool to be left out  because of lack of understanding. Only few people are well versed with this tool (although I did my PhD in this field with eight publications, I never can say that I am strong in this field)

Firstly, I agree with your discussion and

That's why I told that (pl refer to my second answer) in powder form, it is so difficult and because of so many overlapping peaks, you only get broad peak (taking into consideration all possible transitions).But if you do it in single crystalline form, based on orientations, we can easily identify the different transitions. 12 years ago, we did by plotting the theta Vs g or H. (which takes lots and lots of time). Now we have so many software available to do it.

Secondly, I acknowledge you for sharing this topic with me long after.

One solution (a bit time consuming) is the evaluate the electron density distribution and determine the bond critical point properties for the bonded interactions. By calculating the number of bond paths that radiate from each of the bonded atoms, the coordination number of each of the atoms may be determined. See  Bader (199) Atoms in molecules.

Although my answer is maybe a bit late: From the experimental point of view, do an X-ray absorption experiment. This will directly give you the desired quantities. Either from the pre-edge region, where some transitions are dipole forbidden and have a small transition probability, and other are allowed resulting in intense pre-edge peaks (see the above comment of Alexander Ryskin). Or you make use of the extended X-ray absorption fine structure, that will decisively give you the bond distances and coordination numbers - those are different for octahedral and tetrahedral coordinations! You may check details e.g. in

I.J. Pickering, M. Samsone, J. Marsch, G.N. George, J. Am. Chem. Soc. 115 (1993) 6302, where the detailed discrimination is even further elaborated using the anomalous diffraction fine structure.

Do not hesitate to contact me if you need further input, Dirk