The La2S3–LaS2 system: Thermodynamic and kinetic study

A detailed thermodynamic study of the LaS2–La2S3 system in the temperature range 350–1000 °C was performed, starting from high quality crystals LaS2 as the highest polysulfide in the system, and using a sensitive static tensimetric method with a quartz Bourdon gauge and a membrane as a null-point instrument. The pS–T–x diagram obtained has shown that the phase region covering the composition between LaS2 and La2S3, which was previously described as a single grossly nonstoichiometric phase, consists of three discrete stoichiometric phases, LaS2.00, LaS1.91, and LaS1.76, where compositions could be determined with an accuracy of ±0.01 f.u. The thermodynamic characteristics of evaporation of the polysulfides as well as standard heat of LaS2 formation were calculated. The role of kinetics in the formation of ordered superstructures of sulfur-poorer polysulfides with different formal concentration of vacancies is considered.