System Dy-Fe-O: thermodynamic properties of ternary oxides using Calvet calorimetry and solid-state electrochemical cell

Abstract

The enthalpy increments and the standard molar Gibbs energies of formation of DyFeO₃(s) and Dy₃Fe₅O₁₂(s) have been measured using a Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. A co-operative phase transition, related to anti-ferromagnetic to paramagnetic transformation, is apparent from the heat capacity data for DyFeO₃ at ∼648 K. A similar type of phase transition has been observed for Dy₃Fe₅O₁₂ at ∼560 K which is related to ferrimagnetic to paramagnetic transformation. Enthalpy increment data for DyFeO₃(s) and Dy₃Fe₅O₁₂(s), except in the vicinity of the second-order transition, can be represented by the following polynomial expressions: {H°_m(T)-H°_m(298.15K)} (J mol^-1) (±1.1%)=-52754+142.9x(T (K))+2.48x10^-3x(TK))²+2.951x10⁶x(T (K))^-1;(298.15≤T(K)≤1000) for DyFeO₃(s), and {H°_m(T)-H°_m(298.15K)} (J mol^-1) (±1.2%)=-191048+545.0x(T (K))+2.0x10^-5x(T (K))²+8.513x10⁶x(T (K))^-1;(298.15≤T(K)≤1000) for Dy₃Fe₅O₁₂(s). The reversible emfs of the solid-state electrochemical cells: (−)Pt/{DyFeO₃(s) + Dy₂O₃(s) + Fe(s)}//YDT/CSZ//{Fe(s) + Fe_0.95O(s)}/Pt(+) and (−)Pt/{Fe(s) + Fe_0.95O(s)}//CSZ//{DyFeO₃(s) + Dy₃Fe₅O₁₂(s) + Fe₃O₄(s)}/Pt(+), were measured in the temperature range from 1021 to 1250 K and 1035 to 1250 K, respectively. The standard Gibbs energies of formation of solid DyFeO₃ and Dy₃Fe₅O₁₂ calculated by the least squares regression analysis of the data obtained in the present study, and data for Fe_0.95O and Dy₂O₃ from the literature, are given by: Δ_fG°_m(DyFeO₃,s) (J mol-1) (±3.2)=-1339.9+0.2473x(T (K)); (1021≤ T (K)≤ 1548) and Δ_fG°_m(Dy₃Fe₅O₁₂,s) (J mol-1) (±3.5)=-4850.4+0.9846x(T (K)); (1035≤ T(K)≤ 1250). The uncertainty estimates for View the Δ_fG°_m include the standard deviation in the emf and uncertainty in the data taken from the literature. Based on the thermodynamic information, oxygen potential diagram and chemical potential diagrams for the system Dy-Fe-O were developed at 1250 K.