Structural and thermodynamic aspects in the molybdenum melts of the system KF–K2MoO4–SiO2

Abstract

The complex physico-chemical analysis of the system KF–K 2 MoO 4 –SiO 2 , based on the phase diagram, density, surface tension and viscosity measurements, was performed. The phase diagrams of the binary systems as well as that of the ternary system in the range up to 50% SiO 2 were measured using the thermal analysis method. The results proved the presence of the congruently melting compound K 3 FMoO 4 at 751 °C in the binary melt KF–K 2 MoO 4 . The strong positive deviation from ideal behaviour of the ternary system KF–K 2 MoO 4 –SiO 2 was ascribed to the formation of heteropolyanions [SiMo 12 O 40 ] 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 probable inaccuracy in the calculated ternary phase diagram is 5.9 °C. The density of the melts of the system KF–K 2 MoO 4 –SiO 2 was measured using the Archimedean method. Based on the obtained density data the molar volume, partial molar volume and excess molar volume of the melts were calculated in order to consider the possible chemical interactions of components. The degree of thermal dissociation of the additive compound K 3 FMoO 4 was calculated based on both the thermodynamic analysis of the phase diagram and the volume properties of the investigated system. It was found that the degree of the thermal dissociation of K 3 FMoO 4 at the melting point attains the value α o =0.81. The dissociation enthalpy, calculated on the basis of the density values is Δ H (dis, K 3 FMoO 4 )=18.8 kJ mol −1 . The surface tension of the molten system KF–K 2 MoO 4 –SiO 2 has been determined using the maximum bubble pressure method. Based on the obtained data the concentration dependence of the surface tension and of the surface tension excess of the investigated system was calculated. The viscosity of melts of the system KF–K 2 MoO 4 –SiO 2 has been measured using the computerised torsional pendulum method. The sum of viscosities of pure components in logarithm values multiplied by their mole fraction was used as an ‘additive’ behaviour. A multiple linear regression analysis was performed to obtain coefficients of the modified Redlich–Kister's type equation. Binary and ternary interactions of the particular components were found. X-ray powder diffraction patterns and IR spectra of the certain quenched powdered samples of both binary and ternary mixtures were recorded. By both techniques the presence of starting compounds and the presence of another compounds, as well, was detected. Some of these compounds were recognized based on the known data. The remaining data were attributed to the possible expected compounds.