Equilibria in the V2O3-VO2 System at 1600°K

Phase equilibria in the V2O3-VO2 system were established at 1600°K by varying the oxygen partial pressures from 10−13 to 10−3atm. The phases of V2O3, V3O5, V4O7, V5O9, V6O11 and VO2 are stable under these conditions. The only phases of V2O3 and V6O11 have extensive ranges of solid solution. The stability ranges of vanadium oxides with respect to the oxygen partial pressure were determined: V2O3, from 10−13 to 10−6.36atm; V3O5, from 10−6.36 to 10−4.90atm; V4O7, from 10−4.90 to 10−4.20atm; V5O9, from 10−4.20 to 10−4.04atm; and V6O11 from 10−4.04 to 10 −3.53atm, while VO2 is in equilibrium with V6O11 at the oxygen partial pressure of 10−3.53atm. The oxygen partial pressures were measured accurately by means of a solid electrolyte cell composed of (ZrO2)0.85(CaO)0.15; these pressures were then compared with the values calculated from the thermochemical data. On the basis of the equilibrium oxygen partial pressures, the standard free energies of oxidation of various vanadium oxides were calculated.