Gibbs energies of formation of rare earth MPt₅ compounds

Abstract

The Gibbs energies of formation of MPt₅ (MNd, Dy, Ho, Er) intermetallic compounds were determined in the temperature range 900–1100 K using the solid state cell Ta,M+MF₃¦CaF₂¦MPt₅+Pt+MF₃,Ta For M ≡ Sm, a mixture of Gd + GdF₃ was used as the reference electrode. In the case of Eu, a mixture of Eu + EuF₂ served as the reference electrode. The trifluorides of Sm and Eu are not stable in equilibrium with the metal. The fluoride phase coexisting with a SmPt₅ + Pt mixture is SmF₃, whereas EuF₂ is the equilibrium phase in contact with EuPt₅ + Pt. All the MPt₅ compounds studied (except EuPt₅) exhibit similar stability. Europium is divalent in the pure metal and trivalent in EuPt₅. The energy required for the promotion of divalent Eu to the trivalent state accounts for the less negative Gibbs energy of formation of EuPt₅. The enthalpies of formation of all the MPt₅ compounds obtained in this study are in good agreement with Miedema's model.