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 DyFeO 3(s) and Dy 3Fe 5O 12(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 DyFeO 3 at ∼648 K. A similar type of phase transition has been observed for Dy 3Fe 5O 12 at ∼560 K which is related to ferrimagnetic to paramagnetic transformation. Enthalpy increment data for DyFeO 3(s) and Dy 3Fe 5O 12(s), except in the vicinity of the second-order transition, can be represented by the following polynomial expressions: {H 0 m (T)−H 0 m (298.15 K)} ( J mol −1) (±1.1%)=−52754+142.9×(T ( K))+2.48×10 −3×(T ( K)) 2+2.951×10 6×(T ( K)) −1;(298.15⩽ T ( K)⩽1000) for DyFeO 3(s), and {H 0 m (T)−H 0 m (298.15 K)} ( J mol −1) (±1.2%)=−191048+545.0×(T ( K))+2.0×10 −5×(T ( K)) 2+8.513×10 6×(T ( K)) −1;(298.15⩽T ( K)⩽1000) for Dy 3Fe 5O 12(s). The reversible emfs of the solid-state electrochemical cells: (−)Pt/{DyFeO 3(s) + Dy 2O 3(s) + Fe(s)}//YDT/CSZ//{Fe(s) + Fe 0.95O(s)}/Pt(+) and (−)Pt/{Fe(s) + Fe 0.95O(s)}//CSZ//{DyFeO 3(s) + Dy 3Fe 5O 12(s) + Fe 3O 4(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 DyFeO 3 and Dy 3Fe 5O 12 calculated by the least squares regression analysis of the data obtained in the present study, and data for Fe 0.95O and Dy 2O 3 from the literature, are given by: Δ f G 0 m ( DyFeO 3, s) ( kJ mol −1) (±3.2)=−1339.9+0.2473×(T ( K));(1021⩽T ( K)⩽1548) and Δ f G 0 m ( Dy 3 Fe 5 O 12, s) ( kJ mol −1) (±3.5)=−4850.4+0.9846×(T ( K));(1035⩽T ( K)⩽1250). The uncertainty estimates for Δ f G 0 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.