Solid-state immiscibility and thermodynamics of the calcium oxide strontium oxide system

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 Delta G(E) = x(1 - x)[(27040 - 5.82T)x + (24915 - 5.34T)(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.