Thermodynamics of cobalt (II, III) oxide (Co3O4): evidence of phase transition

Kale, G. M. ; Pandit, S. S. ; Jacob, K. T. (1988) Thermodynamics of cobalt (II, III) oxide (Co3O4): evidence of phase transition Transactions of the Japan Institute of Metals, 29 (2). pp. 125-132. ISSN 1345-9678

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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)ZrO2CoO+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: ΔG°=-205,685+170.79T(±200) J mol-1(730-1080 K) and ΔG°=-157,235+127.53T(±200) J mol-1(1150-1250 K).

Item Type:Article
Source:Copyright of this article belongs to Japan Institute of Metals.
Keywords:Spin Transition; Cation Distribution; Gibbs' Energy of Formation; Entropy; Enthalpy; Electrical Conductivity; Lattice Paramete; Differential Thermal Analysis (DTA); Thermogravimetric Analysis (TGA)
ID Code:94844
Deposited On:04 Oct 2012 11:48
Last Modified:04 Oct 2012 11:48

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