Thermodynamic properties and high-temperature behavior of lanthanum selenides La3Se4-La2Se3

The enthalpy of the La 3Se 4 and La 2Se 3 boundary compositions of the La 3-xSe 4 phase (0 ≤ x ≤ 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-xSe 4 selenides in the homogeneity region at 298 K ≤ T ≤ 2123 K. The enthalpy function (J/mole) of La 3-xSe 4 is given by H(T) - (298 K) = (3837 • 10 3T -1 - 85852 + 266.925 • T - 8.7503 • 10 -2 T 2 + 3.437 • 10 -5 T 3) • e -0.2869x. The formation and melting enthalpies of La 3Se 4 are determined: Δ fH °(298 K) = -1340 ± ± 28 kJ/mole), Δ mH = 147.5 ± 9.6 kJ/mole. It is shown that the high-temperature stability of different La 3-xSe 4 compositions is strongly dependent on pressure: as pressure decreases and temperature increases, the selenium content is reduced and the composition tends to La 3Se 4. © 2008 Springer Science+Business Media, Inc.