You can prepare Mn2O3 from MnO2 and MnO (or MnCO3) by annealing at 900 degree C in vacuum.

Mn2O3= MnO+MnO2

MnO2+MnCO3= Mn2O3 + CO2

I agree with Samar

Thank you Samar but I don have Vaccum cahmber furnace. is there any other way to do it.

As you want to prepare RMnO3 perovskites in Poly-crystalline form then you can use MnO2 and R in oxide form, use them in stichiometric proportions and you can use the Solid State Reaction process. No need to separately prepare Mn2O3.

Mn2O3 can be obtained from an aqueous solution containing Mn3 + ion which was added an aqueous solution of ammonia (NH4 +) which gives a precipitate of Mn(OH)3

The obtained product must be annealed at a temperature than higher 100 ° C to obtain the final product Mn2O3

The chemical equations for these reactions are:

Mn 3 + +3 OH---------------> Mn (OH) 3

2mn (OH) 3 ----------------> Mn2O3 +3 H2O

Thank you sir for response. But I have precursors of Mn2+ annd Mn4+ only. that is why I am asking about the conversion process.

R2O3+MnO+MnO2= 2RMnO3

This reaction will work under inert atmosphere or under flowing pure N2 gas

Calcine MnCO3 in air at 500 C or above and pure Mn2O3 will be obtained