Thermodynamic properties and high-temperature behavior of lanthanum selenides La sub(3)Se sub(4)-La sub(2 )Se sub(3)

The enthalpy of the La sub(3)Se sub(4) and La sub(2)Se sub(3) boundary compositions of the La sub(3-x)Se sub(4) phase (0 less than or equal to x less than or equal to 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 sub(3-x)Se sub(4) selenides in the homogeneity region at 298 K less than or equal to T less than or equal to 2123 K. The enthalpy function (J/mole) of La super(3-x)Se sub(4) is given by H(T) - (298 K) = (3837 . 10 super(3)T super(-1) - 85852 + 266.925 . T - 8.7503 . 10 super(-2) T super(2) + 3.437 . 10 super(-5) T super(3)) . e super(-0.2869x). The formation and melting enthalpies of La sub(3)Se sub(4) are determined: Delta sub(f)H degree (298 K) = -1340 +/- +/- 28 kJ/mole), Delta sub(m)H = 147.5 +/- 9.6 kJ/mole. It is shown that the high-temperature stability of different La sub(3-x)Se sub(4) compositions is strongly dependent on pressure: as pressure decreases and temperature increases, the selenium content is reduced and the composition tends to La sub(3)Se sub(4).