Thermodynamic properties of SmFeO₃(s) and Sm₃Fe₅O₁₂(s)

Abstract

The enthalpy increments and the standard molar Gibbs energy (G) of formation of SmFeO₃(s) and Sm₃Fe₅O₁₂(s) have been measured using a Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. A λ-type transition, related to magnetic order-disorder transformation (antiferromagnetic to paramagnetic), is apparent from the heat capacity data at ∼673 K for SmFeO₃(s) and at ∼560 K for Sm₃Fe₅O₁₂(s). Enthalpy increment data for SmFeO₃(s) and Sm₃Fe₅O₁₂(s), except in the vicinity of λ-transition, can be represented by the following polynomial expressions: {H°_m(T)−H°_m(298.15K){/Jmol⁻¹(±1.2%)=−54532.8+147.4⋅(T/K)+1.2⋅10⁻⁴⋅(T/K)2 +3.154⋅106⋅(T/K)−1;(298.15≤T/K≤1000) for SmFeO₃(s), and {H°_m(T)−H°_m(298.15K)}/Jmol⁻¹(±1.4%)=−192763+554.7⋅(T/K)+2.0⋅10−6⋅(T/K)2 +8.161⋅106⋅(T/K)−1;(298.15≤T/K≤1000) for Sm₃Fe₅O₁₂(s). The reversible emf of the solid-state electrochemical cells, (−)Pt/{SmFeO₃(s)+Sm₂O₃(s)+Fe(s)} // YDT / CSZ // {Fe(s)+Fe_0.95O(s)} / Pt(+) and (−)Pt/{Fe(s)+Fe_0.95O(s)} // CSZ // {SmFeO₃(s)+Sm₃Fe₅O₁₂(s)+Fe₃O₄(s)} / Pt(+), were measured in the temperature ranges of 1005–1259 K and 1030–1252 K, respectively. The standard molar G of formation of solid SmFeO₃ and Sm₃Fe₅O₁₂ calculated by the least squares regression analysis of the data obtained in the current study, and data for Fe0.95O and Sm2O3 from the literature, are given by: Δ_fG°_m(SmFeO₃,s)/kJ.mol^-1(±2.0)=-1355.2+02643(T/K);(1005≤T/K≤1570) and Δ_fG°_m0(Sm₃Fe₅O₁₂,s)/kJ⋅mol⁻¹(±3.1)=−4891.0+1.0312⋅(T/K);(1030≤T/K≤1252) The uncertainty estimates for Δ_fG°_m include the standard deviation in the emf and uncertainty in the data taken from the literature. Based on these thermodynamic data, the oxygen potential diagram for the system Sm-Fe-O was constructed at 1250 K.