Formation enthalpies of silicides in Sc-Si system

Formation enthalpies of silicides in the Sc-Si system have been measured by the methods of dissolution in hydrochloric acid, burning in compressed oxygen, and also by the method of indirect fluoranation during joint during of solicides with polytetra-fluoroethylene in compressed oxygen. The following values of standard formation enthalpies of stoichiometric silicides are obtained: (ΔH298 deg kcal/mol):Sc5Si3 - 308.0; Sc30-56.0 and Sc3Si5-136.0. On the basis of analysis of the diagram of g-atomic formation enthalpies the conclusion on the constant valency state of scandium atom in crystal silicides of different compositions is made. In passing the revision of standard formation enthalpy of Sc2O3 is made, which proved to be equal to - 400 Kcal/mol