Thermodynamic Investigation on the Mn-V-O Oxide System and Gibbs Energy of Formation of MnV2O4 Spinel Solid Solution

As the first step of thermodynamic investigation on the Mn-V-O oxide system, phase equilibria of the Mn-V-O systems under controlled atmosphere have been experimentally investigated in the temperature range from 1200 to 1600°C and in the range of logpO2 from -6 to -13. The following phases have been observed; molten oxide, manganosite (MnO (s.s.)), spinel (MnV2O4 (s.s.)), and vanadium trioxide (V2O3 (s.s.)). It was found in the present study that oxygen partial pressure has a significant effect on sub-solidus phase equilibria, in particular for solubility limit of intermediate phase (e.g., MnV2O4 (s.s.)). Once the phase equlibria of the Mn-V-O system was obtained, a subsequent thermodynamic investigation for the MnV2O4 spinel phase was followed to measure the Gibbs energy of formation of the spinel phase employing chemical equilibration technique. The standard Gibbs free energy of formation of the MnV2O4 spinel phase was calculated from the equilibrium compositions of elements in liquid iron or liquid copper, and thermodynamic parameters available in the literatures. From the information obtained in the present study, a stability diagram of Fe-Mn-V-O system was constructed which can be used as an inclusion stability diagram.