Heat of formation of lanthanum-lead alloys

In the La? Pb system the heats of formation were measured, using a differential direct isoperibol calorimeter, and checking composition and state of the samples by chemical, metallographic and X-ray analyses. For the various compounds the _H (for the reaction among solid phases at 300 K) correspond to the following values (Kcal/g · at.): La₅Pb₃ = ?17.2₅ ± 0.5, La₄Pb₃ = ?16.7 ± 0.5, La₅Pb₄ = ?16.5 ± 0.5, La₃Pb₄ = ?15.3 ± 0.5, LaPb₂ = ?14.4 ± 0.5, LaPb₃ = ?13.7 ± 0.5. The results obtained are briefly discussed and compared with those calculated using the method suggested by MIEDEMA .