Dear all
I'm working with graphene \LaMnO3 nanocomposite . my question is :
Is it easy to do calcination in vacuum oven (tube furnace), in order to avoid inducing defects in the graphene layers ( when the composite is calcined in oxygen atmosphere).
The second question is
Why calcination of perovskite should be done in oxygen atmosphere.
I should say that calcination must be performed at the temperatures higher than the temperature of hidroxides decomposition and, second, calcination in vacuum will result in the reduction of manganese to Mn(II). You vill never obtain reproducible stoichiometric LaMnO3/ I do not think even that you will obtain a single phase system.
Depending on temperature and oxygen partial pressure LaMnO3 exhibits various non stoichiometry. Samples prepared in air often present an oxygen excess giving an effective formula close to LaMnO3.1 . On the other hand, oxygen deficient samples can be prepared. For some compositions, oxygen vacancies ordered, F. Abbatista and L. Borlera (International Ceramics 7 (1981) 137) identified the formation of La8Mn8O23 and La4Mn4O11 compounds under moderately reducing conditions. Under more reducing conditions, the non stoichiometry is limited and the compound decomposes to give MnO and La2O3 (same reference and A.M. De Leon-Guevarra et al., J. Alloys and Comp. 262-263 (1997) 163) .
According to J. Töpfer and J.B. Goodenough (J. Solid State Chem. 130 (1997) 117) a stoichiometric composition may be obtained in argon at 1200°C. Ordinary argon contains generally small amount of oxygen giving an oxygen partiel pressure close to 10-6 atm.
I believe that after some optimization (temperature, pressure), conditions allowing the formation of the desired composite can be found.
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