Thermodynamic properties of LaFeO_3−δ and LaFe₁₂O₁₉

Abstract

The standard Gibbs energies of formation of lanthanum orthoferrite (LaFeO_3−δ) and hexaferrite (LaFe₁₂O₁₉) were determined using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen gas at ambient pressure as the reference electrode. From emf of the solid-state cell, the Gibbs energy of formation of nonstoichiometric orthoferrite (LaFeO_3−δ) is obtained. To derive values for the stoichiometric phase, variation of the oxygen nonstoichiometric parameter δ with oxygen partial pressure was measured using thermogravimetry under controlled gas mixtures. The results obtained for LaFeO₃ and LaFe₁₂O₁₉ can be summarized by the following equations, which represent the formation of ternary oxides from their component binary oxides: ½La₂O₃ + ½Fe₂O₃ → LaFeO₃; ΔG° (LaFeO₃) (±450) (J mol⁻¹) = −62920 − 2.12T (K), and ½La₂O₃ + 9/2Fe₂O₃ + Fe₃O₄ →LaFe₁₂O₁₉; ΔG° (LaFe₁₂O₁₉) (±200) (J mol⁻¹) = −103900 + 21.25T (K). These data are discussed critically in comparison with thermodynamic values reported in the literature from a variety of measurements. The values obtained in this study are consistent with calorimetric entropy and enthalpy of formation of the perovskite phase and with some of the Gibbs energy measurements reported in the literature. For the lanthanum hexaferrite (LaFe₁₂O₁₉) there are no prior thermodynamic measurements for comparison.