Gibbs energies of formation of rare earth $MPt_5$ compounds

The Gibbs energies of formation of $MPt_5 (M\equiv$ Nd, Dy, Ho, Er) intermetallic compounds were determined in the temperature range 900–1100 K using the solid state cell Ta,$M+MF_3|CaF_2|MPt_5+Pt+MF_3,$Ta For $M \equiv$ Sm, a mixture of $Gd + GdF_3 $ was used as the reference electrode. In the case of Eu, a mixture of $Eu + EuF_2 $ 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 $SmPt_5 + Pt$ mixture is $SmF_3,$ whereas $EuF_2$ is the equilibrium phase in contact with $EuPt_5 + Pt$. All the $MPt_5$ compounds studied (except $EuPt_5)$ exhibit similar stability. Europium is divalent in the pure metal and trivalent in $EuPt_5$. The energy required for the promotion of divalent Eu to the trivalent state accounts for the less negative Gibbs energy of formation of $EuPt_5$. The enthalpies of formation of all the $MPt_5$ compounds obtained in this study are in good agreement with Miedema's model