Thermodynamic Properties and High-Temperature Behavior of Lanthanum Selenides La₃Se₄–La₂Se₃

Abstract

The enthalpy of the La3Se4 and La2Se3 bounding compositions of the La_3–хSe₄ phase (0 ≤ х ≤ 1/3) at temperatures from 370 to 2260 K is studied calorimetrically. The values obtained are used and the equilibria of the La–Se system are analyzed to establish, for the first time, the temperature–concentration relationships for the thermodynamic properties of La_3–хSe₄ selenides in the homogeneity region at 298 ≤ Т ≤ 2123 K. The enthalpy function (J/mole) of La_3–хSe₄ is given by H(Т) – (298 K) = (3837 · 10³Т^-1 – 85852 + 266,925 · Т – 8,7503 · 10^-2T² + 3,437 · 10^-5T³) · е^–0,2869х. The formation and melting enthalpies of La3Se4 are determined: (∆_f H°(298 K) = –1340 ± 28 (kJ/mole), (∆_mH = 147.5 ± 9.6 (kJ/mole).It is shown that the high-temperature stability of different La_3–хSe₄ compositions is strongly dependent on pressure: as the pressure decreases and the temperature increases, the selenium content is reduced and the composition tends to La₃Se₄.