SOLID-STATE MISCIBILITY GAP AND THERMODYNAMICS OF THE SYSTEM BAO-SRO

A solid-state miscibility gap in the pseudo-binary system BaO-SrO is d elineated by X-ray diffraction studies on samples equilibrated either in vacuum or under flowing inert gas at temperatures between 1073 and 1423 K. For the SrxBa1-xO solid solution an asymmetric phase boundary, characterized by a critical temperature of 1356 (+/-4) K and composit ion x=0.55 (+/-0.008), is obtained. Thermodynamic mixing properties of the solid solution, derived from the experimental phase boundary comp ositions and temperatures, can be represented by the expression: Delta G(E)=x(1-x){33 390-7.09T)x+(29 340-6.23T)(1-x)} J mol(-1) It is neces sary to include excess entropy terms to obtain a good fit to the exper imental data. The chemical spinodal curve is computed from the thermod ynamic parameters.