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:

$$\begin{gathered} \{ H^0 _m (T) - H^0 _m (298.15K)\{ /J mol^{ - 1} ( \pm 1.2\% ) = - 54532.8 + 147.4 \cdot (T/K) + 1.2 \cdot 10^{ - 4} \cdot (T/K)^2 \hfill \\ + 3.154 \cdot 10^6 \cdot (T/K)^{ - 1} ;(298.15 \leqslant T/K \leqslant 1000) \hfill \\ \end{gathered} $$

for SmFeO₃(s), and

$$\begin{gathered} \{ H^0 _m (T) - H^0 _m (298.15K)\} /J mol^{ - 1} ( \pm 1.4\% ) = - 192763 + 554.7 \cdot (T/K) + 2.0 \cdot 10^{ - 6} \cdot (T/K)^2 \hfill \\ + 8.161 \cdot 10^6 \cdot (T/K)^{ - 1} ;(298.15 \leqslant T/K \leqslant 1000) \hfill \\ \end{gathered} $$

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 Fe_0.95O and Sm₂O₃ from the literature, are given by:

$$\Delta _f G^0 _m (SmFeO_3 ,s)/kJ \cdot mol^{ - 1} ( \pm 2.0) = - 1355.2 + 0.2643 \cdot \langle T/K);(1005 \leqslant T/K \leqslant 1570)$$

and

$$\Delta _f G^0 _m (Sm_3 Fe_5 O_{12} ,s)/kJ \cdot mol^{ - 1} ( \pm 3.1) = - 4891.0 + 1.0312 \cdot (T/K);(1030 \leqslant T/K \leqslant 1252)$$

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.