Systems R–Fe–O (R=Ho, Er): Thermodynamic properties of ternary oxides using differential scanning calorimetry and solid-state electrochemical cells

The thermodynamic properties of three different types of ternary oxides RFeO3(s), R3Fe5O12(s) and RFe2O4(s) (where R=Ho and Er) have been determined by calorimetric and solid-state galvanic cell methods. Heat capacities of RFeO3(s) and R3Fe5O12(s) have been determined by differential scanning calorimetry from 130 to 860 K. Heat capacity measurements from 130 to 860 K revealed λ-type anomalies for RFeO3(s) and R3Fe5O12(s) compounds which are assigned due to magnetic order–disorder transitions. The oxygen chemical potentials corresponding to the three-phase equilibria involving these ternary oxides have been determined by using solid-state electrochemical cells. The standard molar Gibbs energies of formation of RFeO3(s), R3Fe5O12(s) and RFe2O4(s) have been computed from the oxygen potential data. Based on the thermodynamic information, oxygen potential diagrams have been computed for the systems R–Fe–O (R  =Ho and Er) at two different temperatures: T=1250T=1250 and 1450 K. Thermodynamic functions like Cp,mo, Smo, Ho, Go, (HTo-H0o), (HTo-H298.15Ko), -(GTo-H298.15Ko)/T, ΔfHmo, and ΔfGmo have been generated for the compounds RFeO3(s) and R3Fe5O12(s) based on the experimental data obtained in this study and the available data in the literature.

Plot of heat capacity against temperature for HoFeO3(s) showing different contributions: (●) total heat capacity; (○) lattice contribution; and (▴) magnetic contribution.Figure optionsDownload as PowerPoint slide