Low temperature thermal and volumetric behavior of MnAl2O4 spinel

The thermodynamic behavior of the pseudo-binary Al2O3 – MnO is of high value for the steel industry. However, its phase diagram has been associated with some controversy regarding the equilibrium conditions involving the spinel phase (MnAl2O4) at higher temperatures. One way to handle this issue would be the development of a more accurate Gibbs energy models for this mixed oxide, which by itself demands high precision heat capacity data. In the present article, Al2MnO4 samples are synthesized at 1373 K under a reduced atmosphere (C/CO) and characterized through different methods: transmission electron microscopy (TEM) and X-ray diffraction (XRD). The volumetric behavior of Al2MnO4 has next been addressed through XRD analysis with synchrotron radiation during cooling between 300 and 20 K. Finally, selected samples have been studied in a relaxation calorimeter for measuring CP in the range between 2 to 300 K. A reversible thermal anomaly has been detected, with a peak centered around 33 K. Such phenomena resulted in a measurable contribution to the molar entropy of the spinel phase (4.7 ± 0.05 J/mol.K). Considering this contribution, the mean molar entropy of Al2MnO4 at 298.15 K has shown to be equal to 118.7 ± 0.5 J/mol.K.