A detailed thermodynamic study of the systems LnSe2–LnSe1.5 (Ln = La, Nd) was performed by static method of vapour pressure measurement using quartz membrane-gauge manometers within the
temperature range 713–1,395 K. The pSe–T–x dependences obtained in this study have shown that the phase regions in composition intervals studied consist of discrete
phases: LnSe1.95LnSe1.90, LnSe1.85, LnSe1.80 (Ln = La, Nd). The enthalpies and the entropies for the stepwise dissociation process were calculated from the experimental data.
The standard enthalpies of formation and the absolute entropies were estimated for the compounds investigated using literature