Systems Ln–Fe–O ( Ln=Eu, Gd): thermodynamic properties of ternary oxides using solid-state electrochemical cells

Abstract

The standard molar Gibbs energies of formation of LnFeO 3(s) and Ln 3Fe 5O 12(s) where Ln=Eu and Gd have been determined using solid-state electrochemical technique employing different solid electrolytes. The reversible e.m.f.s of the following solid-state electrochemical cells have been measured in the temperature range from 1050 to 1255 K. Cell (I): (−)Pt / { LnFeO 3(s)+ Ln 2O 3(s)+Fe(s)} // YDT/CSZ // {Fe(s)+Fe 0.95O(s)} / Pt(+); Cell (II): (−)Pt/{Fe(s)+Fe 0.95O(s)}//CSZ//{ LnFeO 3(s)+ Ln 3Fe 5O 12(s)+Fe 3O 4(s)}/Pt(+); Cell (III): (−)Pt/{ LnFeO 3(s)+ Ln 3Fe 5O 12(s)+Fe 3O 4(s)}//YSZ//{Ni(s)+NiO(s)}/Pt(+); and Cell(IV):(−)Pt/{Fe(s)+Fe 0.95O(s)}//YDT/CSZ//{ LnFeO 3(s)+ Ln 3Fe 5O 12(s)+Fe 3O 4(s)}/Pt(+). The oxygen chemical potentials corresponding to the three-phase equilibria involving the ternary oxides have been computed from the e.m.f. data. The standard Gibbs energies of formation of solid EuFeO 3, Eu 3Fe 5O 12, GdFeO 3 and Gd 3Fe 5O 12 calculated by the least-squares regression analysis of the data obtained in the present study are given by Δ f G° m (EuFeO 3, s) /kJ mol −1 (± 3.2)=−1265.5+0.2687( T/K) (1050 ⩽ T/K ⩽ 1570), Δ f G° m (Eu 3Fe 5O 12, s)/kJ mol −1 (± 3.5)=−4626.2+1.0474( T/K) (1050 ⩽ T/K ⩽ 1255), Δ f G° m (GdFeO 3, s) /kJ mol −1 (± 3.2)=−1342.5+0.2539( T/K) (1050 ⩽ T/K ⩽ 1570), and Δ f G° m (Gd 3Fe 5O 12, s)/kJ·mol −1 (± 3.5)=−4856.0+1.0021( T/K) (1050 ⩽ T/K ⩽ 1255). The uncertainty estimates for Δ f G° m include the standard deviation in the e.m.f. and uncertainty in the data taken from the literature. Based on the thermodynamic information, oxygen potential diagrams for the systems Eu–Fe–O and Gd–Fe–O and chemical potential diagrams for the system Gd–Fe–O were computed at 1250 K.