It is very difficult to achieve stoichiometry for such systems (MFe2O3) by precipitation of hydroxide and calcination of it unless some controlled conditions are used. Fe2O3 is more favorable phase of Fe oxide usually (Thus, gravimetric analyis of Fe is possible). The normal preparation of Ferrites includes Fe2+ and Fe3+ salts and controlled and optimised sintering.. If temperature is not optimised Fe2O3 phase out. You may check following reference which shows the preparation of Fe2O3 by Fe3O4 heating.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.475.580&rep=rep1&type=pdf

More there could be more reasons why you observed an impure phase.

1) Stoichometry was not maintained in practice (M2+/3+ and Fe 2+/3+). Thus some of Fe formed Fe2O3. (Fe could form inverse and normal spinel depending on type and concentration of M)

2) The substitution which is attempted is not forming a pure phase as it violet Hume rothery rules.

3) Ferrite is a solid phase of Fe oxide where Fe3+ and Fe2+ is stabilized. As you observe Fe2O3 i.e. Fe3+ in majorly, you need to stabilize it by adding more of the M or decrease M. M could be 2+, 3+ 4+ etc so there is optimization needed as it changes the concentration of Fe2+ and 3+ in ferrite.

If you are preparing solid solutions of MxFe3-xO4 then this is usually obseved at x< 0.8

Hope this was helpful.

Thank you Mithil. answer is really very helpful. I will work out for synthesis procedure and conditions accordingly.

Hi. You may try the sol-gel method for preparing the ferrites. The so-gel method would produce ferrites of higher quality (purity and crystallinity) compared with the co-precipitation method.

The enclosed paper may help.

Thank you sir

Thank You

I agree with Dr. Mohammad Abu Haija and I have few suggestions here.

This problem is unique in case of some ferrites (MOFe2O3: M=Mn, Mg, Cu ).

I have observed that the though the single spinel phase is formed in as-prepared samples but it is not stable when annealed at temperature range from 600- 1000C which results the formation of secondary phase along with spinel phase.

The formation of secondary phase can be controlled in as-prepared samples by controlling the annealing environments.

Some details are discussed in the below paper.

In your case you may have the secondary phase of Mn2O3 also since you are preparing the MnFe2O4.

The formation of single phase also depends what kinds precursors have been used in particular chemical method.