The DFT-based calculations of silicon complex structures in the KF-KCl-K2SiF6 and KF-KCl-K2SiF6-SiO2 melts

Alexey S. Vorob’ev, Andrey Isakov, Alexander Galashev, Yurii P. Zaikov

Abstract


The length and energy of bonds in complex anions of silicon, formed in the KF−KCl−K2SiF6 and KF−KCl−K2SiF6–SiO2 melts, have been evaluated using method of the first - principles molecular dynamics, accomplished by means of the Siesta program. The effect of the K+ ions (from the second coordination sphere) on the stability of these complexes has been studied. The bond length in the silicon complexes was found to be changed with increasing amount of the potassium ions. It was established that the following complexes [SiO4]4-, [SiO3F]3-, [SiF6]2- are the most stable in the KF−KCl−K2SiF6 and KF−KCl−K2SiF6−SiO2 melts. The [SiO4]4- and [SiF6]2- complexes are thermally stable in the molten salt in the temperature range of 923-1073 K, whereas the [SiF7]3- structure, which is typical for the lattice of crystalline K3SiF7, is unstable in this temperature range. In the KF−KCl−K2SiF6−SiO2 melts, the conditions above 1043 K were created allowing the transformation of [SiО3F]3- into [SiO4]4-.Within the studied temperature mode, the Si-F bond length is in the range of 1.5-1.9 Å, and the Si-O bond lengths is 1.5-1.7 Å. The obtained results are in a good agreement with the data in situ of the Raman spectroscopy for the KF−KCl−K2SiF6 and KF−KCl−K2SiF6−SiO2 melts.


Keywords


molten salts; silicon complexes; structure; DFT calculations

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DOI: https://doi.org/10.2298/JSC190225044V

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