Role of entropy in the stability of cobalt titanates

Abstract

The standard molar Gibbs free energy of formation of Co₂TiO₄, CoTiO₃, and CoTi₂O₅ 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 TiO₂ with rutile structure, the Gibbs free energy changes are given by: Δ_fG°_(ox)(Co₂TiO₄)±104/(J·mol^-1)=-18865-4.108(T/K) Δ_fG°_(ox)(CoTiO₃)±56/(J·mol^-1)=-19627+2.542(T/K) Δ_fG°_(ox)(CoTi₂O₅)±52/(J·mol^-1)=-6223-6.933(T/K) Accurate values for enthalpy and entropy of formation were derived. The compounds Co₂TiO₄ with spinel structure and CoTi₂O₅ 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 CoTiO₃ was confirmed by inert gas fusion analysis for oxygen. Because of partial oxidation of cobalt in air, the composition corresponding to the compound Co₂TiO₄ falls inside a two-phase field containing the spinel solid solution Co₂TiO₄–Co₃O₄ and CoTiO₃. The spinel solid solution becomes progressively enriched in Co₃O₄ with decreasing temperature.