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

Abstract

The thermodynamic properties of three different types of ternary oxides RFeO 3(s), R 3Fe 5O 12(s) and RFe 2O 4(s) (where R=Ho and Er) have been determined by calorimetric and solid-state galvanic cell methods. Heat capacities of RFeO 3(s) and R 3Fe 5O 12(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 RFeO 3(s) and R 3Fe 5O 12(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 RFeO 3(s), R 3Fe 5O 12(s) and RFe 2O 4(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 = 1250 and 1450 K. Thermodynamic functions like C p , m o , S m o , H o, G o, ( H T o - H 0 o ) , ( H T o - H 298.15 K o ) , - ( G T o - H 298.15 K o ) / T , Δ f H m o , and Δ f G m o have been generated for the compounds RFeO 3(s) and R 3Fe 5O 12(s) based on the experimental data obtained in this study and the available data in the literature.