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 Ca_xSr_1-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 ΔG^E = 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.