Isothermal stability field diagrams for Ln−O−Cl systems

Thermodynamic data for Eu3O4 are not available in standard compilations. However, data for EuO and Eu2O3 are available. Data for Eu2O3 in the compilations are in agreement while those for EuO differ significantly. Two solid–state electrochemical cells incorporating yttria–doped thoria as the electrolyte are used to measure the standard Gibbs energy of formation of Eu3O4 and EuO relative to that for Eu2O3 in the temperature range from 1000 to 1300 K. A mixture of Nb and NbO is used as the reference electrode since the oxygen chemical potential associated with the mixture is close to that of the working electrodes EuO + Eu3O4 and Eu3O4 + Eu2O3. The standard Gibbs energy of formation of Eu3O4 can be represented by the following equation: Δf G°(±860)/J·mol–1 = −2267816 + 403.18­(T/K) {1090–1300 K}. Below the melting point of Eu, the Gibbs energy of formation of Eu3O4 is given by Δf G°(±860)/J·mol–1 = −2240177 + 377.824­(T/K) {1000–1090 K}. Also, the standard Gibbs energy of formation of EuO can be represented by the equations: Δf G°(±350)/J·mol–1 = −601046 + 96.878­(T/K) {1000–1090 K} and Δf G°(±350)/J·mol–1 = −610259 + 105.33­(T/K) {1090–1300 K}. Based on these results and the Neumann–Kopp rule, a complete set of thermodynamic data for Eu3O4 is generated. The results also permit refinement of data for EuO.

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