SOLID-STATE IMMISCIBILITY AND THERMODYNAMICS OF THE CALCIUM-OXIDE STRONTIUM OXIDE SYSTEM

Solid state miscibility gap in the pseudo-binary calcium oxide-stronti um oxide (CaO-SrO) system is delineated by X-ray diffractometry studie s on samples equilibrated either under vacuum or in a flowing inert-ga s atmosphere at temperatures of 973-1273 K. For the CaxSr1-xO solid so lution, an asymmetric phase boundary that is characterized by a critic al 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 a nd 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), I t is necessary to include excess entropy terms to obtain a good fit to the experimental data. The chemical spinodal curve is computed from t he thermodynamic parameters. The results obtained in this study are co mpared with phase-diagram and calorimetric information that is availab le in the literature.