Solid‐State Immiscibility and Thermodynamics of the Calcium Oxide‐Strontium Oxide System

Abstract

Solid‐state miscibility gap in the pseudo‐binary calcium oxide‐strontium oxide (CaO‐SrO) system is delineated by X‐ray diffractometry studies on samples equilibrated either under vacuum or in a flowing inert‐gas atmosphere at temperatures of 973‐1273 K. For the CaxSr1−xO solid solution, an asymmetric phase boundary that is characterized by a critical temperature of 1173 (±3) K and a composition of x= 0.53 (±0.01) is obtained. The excess Gibbs energy of mixing of the solid solution, which is derived from the experimental phase‐boundary compositions and temperatures, can be represented by the expression ΔGE=x (1−x)[(27040 − 5.82 T) x+ (24915 − 5.34 T)(1 −x)] (in units of J/mol). It is necessary to include excess entropy terms to obtain a good fit to the experimental data. The chemical spinodal curve is computed from the thermodynamic parameters. The results obtained in this study are compared with phase‐diagram and calorimetric information that is available in the literature.