Phase Diagram Study of the CaO^|^ndash;CaF2 System

Abstract

Key phase diagram experiments for the CaO–CaF2 system were conducted using an equilibration and quenching technique and thermal analysis. Equilibration and quenching experiments using sealed Pt capsules followed by FE-SEM EDS compositional analysis and XRD phase analysis were performed to determine the CaO and CaF2 liquidii and the solubility of CaO in solid CaF2. Thermal analyses with DSC and TGA were used to determine the temperatures of the eutectic and the α- to β-CaF2 polymorphic transition. For the first time, noticeable solubility of CaO in α- and β-CaF2 solids is observed above 1000°C. The maximum solubility of CaO in solid CaF2 is about 5 mol% at the eutectic temperature (about 1361°C) while no solubility of CaF2 in solid CaO is detected. Based on the thermodynamic optimization using the present CaO and CaF2 liquidii and the eutectic temperature measurements, it is concluded that the CaO–CaF2 binary is a simple eutectic system with the eutectic reaction L(CaO = 15 mol%) → CaO(pure solid) + CaF2(solid solution with CaO = 5 mol%) at 1361°C. The α- to β-CaF2 polymorphic transition is located at about 1146°C and the melting point of CaF2 at 1420°C.