Abstract
A detailed thermodynamic study of the LaS 2–La 2S 3 system in the temperature range 350–1000 °C was performed, starting from high quality crystals LaS 2 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 p S– T– x diagram obtained has shown that the phase region covering the composition between LaS 2 and La 2S 3, which was previously described as a single grossly nonstoichiometric phase, consists of three discrete stoichiometric phases, LaS 2.00, LaS 1.91, and LaS 1.76, 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 LaS 2 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.
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