Abstract
The paper explores questions of structural-chemical aspects of peritectic compounds formation in the Ga2S3-Ln2O2S (where Ln-La,Nd) system. It found that these reactions proceed due to inheriting structural peculiarities of the peritectic compound by the solid product of reaction. Specifically, the structure of peritectic compounds and solid products of their decomposition are interrelated by means of alternative, simple plans of reconstruction while the presence of a cooperative mechanism makes the kinetics of transition between them much easier. Note that the structures of all peritectic compounds of these reactions fall into the derivative or composite type.