CALPHAD: Phase diagram of the system KF---K2MoO4---SiO2

The phase diagrams of the binary systems KF---K2MoO4, KF---SiO2, and K2MoO4---SiO2, as well as that of the ternary system KF---K2MoO4---SiO2 in the range up to 50 mole % SiO2, were measured using the thermal analysis method. The thermodynamically consistent phase diagrams were calculated using the coupled analysis of the thermodynamic and phase diagram data. In the system KF---K2MoO4 the intermediate compound K3FMoO4, melting congruently at 751 °C, is formed. This compound divides this system into two simple eutectic ones. The coordinates of the individual eutectic points are: E1: 30.5 mole % K2MoO4, 720.4 °C, and E2: 58.8 mole % K2MoO4, 744.9 °C. In the binary system KF---SiO2 the liquidus curve of KF shows an inflex point, characteristic for reciprocal systems with chemical reaction taking place between components. Similar course of the liquidus curve of K2MoO4 was found in the binary system K2MoO4---SiO2, indicating the presence of the chemical reaction between components as well. The strong positive deviation from ideal behavior of the ternary system KF---K2MoO4---SiO2 was ascribed to the possible formation of heteropolyanions [SiMo12O40]4− in the melt. In the investigated concentration range of the ternary system no eutectic point has been found. It lies most probably beyond the investigated part of the system. The standard deviation of approximation of the calculated ternary phase diagram is ± 5.9 °C, which is approximately on the same level of magnitude as the estimated experimental error of ± 4 °C.