Hi Mkhitar,

As many people have noted, conversion from Fe3O4 to Fe2O3 is possible without reducing agents. The process is called oxygen dissociation and can occur at temperatures > 800-1000C, but the exact temperature varies due to composition and oxygen partial pressure.

As Raja noted above, preserving the magnetite on cooling can be quite tricky. Generally, if you are heating in a vacuum with an active pump (i.e., you are working to maintain the vacuum) the free oxygen will be removed from the environment so on cooling magnetite will be preserved. Otherwise oxygen partial pressure controls the re-uptake of oxygen, but reducing agents help.

A quick hysteresis loop and back field demagnetization curve (to get coercivity and coercivity of remanence, respectively) should give a quick indication of magnetite/maghemite (soft) versus hematite (hard) or a mixture of both.

As a note on distinguishing magnetite and maghemite. It is possible with XRD, but only if the maghemite is highly crystalline. Otherwise it is all but indistinguishable from magnetite. An alternative approach would be low temperature Mossbauer, but this is a technique that I am not completely familiar with. Perhaps someone else can help further.

I, very humbly, can see the possibility of the existence of MgO-Fe3O4 as:

The spinel MgFe2O4 can be thought to be analogous to MgO+ Fe2O3.It is reported in the literature that heating at 1670 K converts Fe2O3 to black Fe3O4 (FeIIFeIII2O4 )[Wikipedia]

Even in the presence of Mg(II)[ present as MgO], there is no possibility of Fe(II) reacting with with Mg(II) because:

[i]Mg has greater affinity for oxygen than iron.

[ii] There is no possibility the following reaction to occur because in that case cell potential will have negative value.

Fe(II)----------------------Fe(III)+e ; E^0= -77V

Mg(II)+ 2e----------------- Mg(0); E^0=- 2.37 V

Multiply first equation by 2 and add

----------------------------------------------------------------

Mg(II)+2Fe(II)-------- Mg(0)+ 2Fe(III),∆E0= -3.14V.

So it can be concluded that MgO-Fe3O4 can exist.

Dear Mkhitar,

It is possible to convert Fe2O3 to Fe3O4 without reducers, by heating it in vacuum only. I found a references in support of this statement (F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Interscience Publishers, New York, 1962, p. 709.) 

Fe2O3 is the most stable compound among the iron oxides and all the other iron oxides gets converted to Fe2O3 easily. As for your case it is quite possible that the mixture contains iron oxide with +3 state, and by heating it gets converted to Fe3O4. It would be premature to confirm it. To be sure about it you can go for a XRD measurement or XPS measurement.

Thank you very much,, Manohar and Surajit, i will look at the reference you privided. Indeed, I am not heating under vacuum, I am heating in ambient air, but the conversion takes place. Good suggestion, XRD might give some hints as well.

Alternatively, the magnetic phase could be maghemite (gamma-Fe2O3). Unfortunately, conventional XRD would not be helpful to identify it, since both magnetite and maghemite have the same cubic structure with similar lattice parameters.

Phase difference between Fe3O4 and y-Fe2O3 or other for  can be identified using x ray photoelectron spectroscopy [XPS ] , using this technique different phase of iron oxides can be identified precisely . Check this paper for more details : Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials , Toru Yamashita,, Peter Hayes, Applied Surface Science , Volume 254, Issue 8, 15 February 2008, Pages 2441–2449.

conversion of fe203 from fe3o4 is always requires some reducing agent at low temperature, however,( in general many first row metal oxide )  when heating at high temperature will release some oxygen there by get reduced , BUT, their stability now depends on the many factors, rate of heating and cooling , partial pressure of oxygen in the working environment, addition matrix in your case MgO (the stability basically depends on reabsorbs of removed oxygen kinetics)  

Hi Mkhitar,

As many people have noted, conversion from Fe3O4 to Fe2O3 is possible without reducing agents. The process is called oxygen dissociation and can occur at temperatures > 800-1000C, but the exact temperature varies due to composition and oxygen partial pressure.

As Raja noted above, preserving the magnetite on cooling can be quite tricky. Generally, if you are heating in a vacuum with an active pump (i.e., you are working to maintain the vacuum) the free oxygen will be removed from the environment so on cooling magnetite will be preserved. Otherwise oxygen partial pressure controls the re-uptake of oxygen, but reducing agents help.

A quick hysteresis loop and back field demagnetization curve (to get coercivity and coercivity of remanence, respectively) should give a quick indication of magnetite/maghemite (soft) versus hematite (hard) or a mixture of both.

As a note on distinguishing magnetite and maghemite. It is possible with XRD, but only if the maghemite is highly crystalline. Otherwise it is all but indistinguishable from magnetite. An alternative approach would be low temperature Mossbauer, but this is a technique that I am not completely familiar with. Perhaps someone else can help further.