Thermodynamic properties of $Pt_5La, Pt_5Ce, Pt_5Pr, Pt_5Tb$ and $Pt_5Tm$ intermetallics

The Gibbs' energies of formation of $Pt_5La, Pt_5Ce, Pt_5Pr, Pt_5Tb$ and $Pt_5Tm$ intermetallic compounds have been determined in the temperature range 870-110C K using the solid state cell: $Ta,M+MF_3/CaF_2/Pt_5M+Pt+MF_3,Ta$. The reversible emf of the cell is directly related to the Gibbs' energy of formation of the $Pt_5M$ compound. The results can be summarized by the equations: relative to the low temperature allotropic form of the lanthanide element and solid platinum as standard states The enthalpies of formation of all the $Pt_5M$ intermetallic compounds obtained in this study are in good agreement with Miedema's model. The experimental values are more negative than those calculated using the model. The variation of the thermodynamic properties of $Pt_5M$ compounds with atomic number of the lanthanide element is discussed in relation to valence state and molar volume