Thermodynamics of Co₃O₄: a possible electron spin unpairing transition in Co³⁺

The free energy of the reaction: {Mathematical expression} has been studied between 890 and 1,240 K using an e.m.f. technique. There is a phase transition in Co₃O₄ at 1,120±20 K which is accompanied by a large change in entropy (∼47 JK^-1 mol^-1 of Co₃O₄), and a rapid increase in unit cell volume and in electical conductivity. This is interpreted to be due to a partial change in electronic spin states in Co^{3 +} from the spin-paired (low spin) configuration observed at room temperature to the spin-unpaired (high spin) state. The transition is probably not first order.