Thermodynamic investigation of the phase formation processes in the systems LnSe2–LnSe1.5 (Ln = La, Ce, Pr, Nd)

The pressure of selenium was measured by the static method with quartz membrane-gauge manometers in the systems LnSe2–LnSe1.5 (Ln = Ce, Pr) within the temperature range (893–1332) K. From pSe-T-x dependences obtained, the composition of intermediate phases and the thermodynamic characteristics (ΔrH298, ΔrS°298) for the stepwise dissociation processes in the systems studied were calculated. Data obtained in our previous works for the systems LnSe2–LnSe1.5 (Ln = La, Pr, Nd) were reprocessed in connection with more correct estimation of heat capacities and absolute entropies of polyselenides. The standard thermodynamic functions (ΔfH298, S°298, ΔfG°298) of La, Ce, Pr, Nd polyselenides were determined. Solid phase compositions in the systems LnSe2–LnSe1.5 (Ln = La, Ce, Pr, Nd) may be described by general formula LnnSe2n−1 (n = 5, 7, 10, 20). The dependence of thermodynamic values of polyselenides on the selenium content was analysed. The set of thermodynamic characteristics obtained may be used for phase equilibria calculation and for thermodynamic modelling of single-crystal growth processes in the systems LnSe2–LnSe1.5 (Ln = La, Ce, Pr, Nd).

► Pressure of selenium was measured by static method in systems LnSe2–LnSe1.5 (Ln = Ce, Pr).
► Composition of phases and thermodynamic values for dissociation processes were calculated.
► The standard thermodynamic functions of La, Ce, Pr, Nd polyselenides were determined.
► The dependence of thermodynamic values of polyselenides on selenium content was analysed.