Role of entropy in the stability of cobalt titanates

The standard molar Gibbs free energy of formation of Co 2TiO 4, CoTiO 3, and CoTi 2O 5 as a function of temperature over an extended range (900 to 1675) K was measured using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte, with CoO as reference electrode and appropriate working electrodes. For the formation of the three compounds from their component oxides CoO with rock-salt and TiO 2 with rutile structure, the Gibbs free energy changes are given by: Δ f G ( ox ) ∘ ( Co 2 TiO 4 ) ± 104 / ( J · mol - 1 ) = - 18865 - 4.108 ( T / K ) Δ f G ( ox ) ∘ ( CoTiO 3 ) ± 56 / ( J · mol - 1 ) = - 19627 + 2.542 ( T / K ) Δ f G ( ox ) ∘ ( CoTi 2 O 5 ) ± 52 / ( J · mol - 1 ) = - 6223 - 6.933 ( T / K ) Accurate values for enthalpy and entropy of formation were derived. The compounds Co 2TiO 4 with spinel structure and CoTi 2O 5 with pseudo-brookite structure were found to be entropy stabilized. The relatively high entropy of these compounds arises from the mixing of cations on specific crystallographic sites. The stoichiometry of CoTiO 3 was confirmed by inert gas fusion analysis for oxygen. Because of partial oxidation of cobalt in air, the composition corresponding to the compound Co 2TiO 4 falls inside a two-phase field containing the spinel solid solution Co 2TiO 4–Co 3O 4 and CoTiO 3. The spinel solid solution becomes progressively enriched in Co 3O 4 with decreasing temperature.