The Gibbs’ energy change for the reaction, 3CoO (r.s.)+1/2O₂(g)→Co₃O₄(sp), has been measured between 730 and 1250 K using a solid state galvanic cell: Pt, CuO+Cu₂O|(CaO)ZrO₂|CoO+Co₃O₄,Pt. The emf of this cell varies nonlinearly with temperature between 1075 and 1150 K, indicating a second or higher order phase transition in Co₃O₄around 1120 (±20) K, associated with an entropy change of ∼43 Jmol⁻¹K⁻¹. The phase transition is accompanied by an anomalous increase in lattice parameter and electrical conductivity. The cubic spinel structure is retained during the transition, which is caused by the change in CO⁺³ ions from low spin to high spin state. The octahedral site preference energy of CO⁺³ ion in the high spin state has been evaluated as −24.8 kJ mol⁻¹. This is more positive than the value for CO⁺² ion (−32.9 kJ mol⁻¹). The cation distribution therefore changes from normal to inverse side during the phase transition. The transformation is unique, coupling spin unpairing in CO⁺³ ion with cation rearrangement on the spinel lattice, DTA in pure oxygen revealed a small peak corresponding to the transition, which could be differentiated from the large peak due to decomposition. TGA showed that the stoichiometry of oxide is not significantly altered during the transition. The Gibbs’ energy of formation of Co₃O₄ from CoO and O₂ below and above phase transition can be represented by the equations:
ΔG⁰=−205,685+170.79T(±200)J mol⁻¹(730–1080 K)
and
ΔG⁰=−157,235+127.53T(±200)J mol⁻¹(1150–1250 K).