PHASE-EQUILIBRIA IN THE LA-MO-SE SYSTEM AT 1200 DEGREES-C IN THE VICINITY OF LAMO6SE6 AND MO3SE4

Phase relations prevalent in the La-Mo-Se system at 1200 degrees C in the vicinity of the ternary (Chevrel) phase LaMo6Se8 and of the binary Mo3Se4 phase, have been established. Characteristic features of this system are as follows: (i) Two isostructural, but mutually immiscible, solid solutions covering considerable compositional intervals exist. One of them, defined as La-intercalated Mo3Se4, spreads over a triangu lar surface, with its origin at the binary Mo3Se4 and developing into the ternary system, as far as approximate to 1 at% La. One edge of thi s triangle points towards the isostructural ternary phase LaMo6Se8. Th e second three-component solid solution is based on the Chevrel-type p hase LaMo6Se8 and takes the shape of a rectangular surface about 2 at% La wide, with 6.67 at% La as the mean value. The domain runs along th e line LaMo6+xSe8-x, from x = 0 (40 at% Mo), characteristic of the exa ct stoichiometry 1:6:8, to x = 0.75 (45 at% Mo). (ii) The continuous s olid solution of the LaxMo6Se8 type (0 less than or equal to x less th an or equal to 1), expected to occur between the binary Mo3Se4 and the ternary LaMo6Se8, is absent. The domains of both solid solutions have been established using x-ray powder diffractometry and the evolution of the critical temperatures (T-c) characteristic for each of the phas es.