Phase Diagram Study of the CaO–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)} + CaF_{2(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.