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The effect of MgO on the microstructures and macroscopic properties of SiO2‐poor/rich MgO‐CaO‐SiO2 melts was investigated by in situ high temperature time‐gated Raman spectroscopy coupled with aerodynamic levitation heating of molten samples. Results of density functional theory calculations of Mg2SiO4, γ‐Ca2SiO4, and MgCaSiO4 crystals and quantum chemistry ab initio simulations of Q0 species (Q0 refers to tetrahedral Si‐O units without bridging oxygen) of SiO2‐poor series MgO‐CaO‐SiO2 molten samples certified that with the substitution of CaO by MgO, the variation of Raman intensity at 900–1100 cm−1 of xMgO‐(2‐x)CaO‐SiO2 melts was caused by the antisymmetric stretching vibration modes of distorted silicon‐oxygen tetrahedra. The experimental result of SiO2‐rich series of MgO‐CaO‐SiO2 molten samples could identify various Qi species quantitatively and demonstrated that with the substitution of CaO by MgO, the abundance of Q3 species dramatically decreased, but Q4 species increased slightly, whereas Q1 and Q2 species maintain insignificant change. A positive correlation between viscosity and the abundance ratio of Q3 to Q4 was assumed and illustrated. [ABSTRACT FROM AUTHOR]

In order to help predict the properties of molten salts electrolytes and aluminium brazing flux used in the Hall–Héroult process and in aluminium brazing, the microstructure of binary KF‐AlF3 system from 298 K to its molten states was explored by in situ high‐temperature Raman spectroscopy in argon atmosphere. Comparing with experimental Raman spectra in molten system, quantum chemistry ab initio simulations were used to calculate the symmetric stretching vibrational modes of probable clusters in the melts. The molten KF‐AlF3 system was found to comprise monomer [AlF6]3−, [AlF5]2−, [AlF4]−, and dimer [Al2F7]− clusters. With the increase of KF content in the melts, the major species changes from [AlF4]− tetrahedra to [AlF6]3− octahedra. From the deconvoluted measured Raman spectra, the mole fractions of varied clusters in the system were also calculated. As the KF:AlF3 ratio was increased from 1 to 3, the content of [AlF6]3− in the melt increased from 22.30 to 98.83 mol%, whereas the [AlF4]− content decreased from 68.92 to 0.33 mol%. [AlF5]2− and [Al2F7]− clusters were also found in small amounts in the melts. [ABSTRACT FROM AUTHOR]

In‐situ studies of the vibrational characteristics and microstructure evolution of the Na2WO4·2H2O (sodium tungstate dihydrate) crystal during the temperature‐induced solid‐state phase transformation and melting process were carried out using high‐temperature Raman spectroscopic technique. Results showed that the thermal decomposition process of the Na2WO4·2H2O crystal takes place mainly within the temperature range of 348–383 K, along with the structure transforming from the orthorhombic to cubic symmetry. As the sample temperature increased further, another solid‐state phase transformation from the cubic to orthorhombic structure was observed approximately at 893 K before melting occurred at 1023 K. Although the isolated [WO4]2− tetrahedron was preserved within the entire temperature range from room temperature to 1023 K, subtle changes were observed with the mean bond length of W–O bonds in the tetrahedron unit. Furthermore, Raman active vibrational modes of Na2WO4·2H2O, two Na2WO4 crystal phases, and corresponding melt were assigned based on the density functional theory simulation and compared with the literature data. Finally, four‐molecule cluster arranged as Td symmetry is considered to be the most likely configuration in the molten state according to density functional theory simulation based on the different multimolecular clusters proposed. In‐situ Raman spectra of the polycrystalline Na2WO4·2H2O from room temperature to 1023 K were investigated systematically. The vibrational modes of Na2WO4·2H2O, two Na2WO4 crystal phases, and corresponding melt were assigned based on density functional theory simulation. Microstructure evolution of [WO4]2− during dehydration, solid‐state phase transformation, and melting process was evaluated. Several multimolecular cluster models were constructed, and their Raman active vibrational modes were simulated in order to determine the most likely arrangement of [WO4]2− tetrahedra in the molten state. [ABSTRACT FROM AUTHOR]