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

Abstract

The length and energy of bonds in the complex anions of silicon formed in KF−KCl−K₂SiF₆ and KF−KCl−K₂SiF₆–SiO₂ melts were evaluated using the method of first-principles molecular dynamics, accomplished by means of the Siesta program. The effect of K⁺ (from the second coordination sphere) on the stability of these complexes was studied. The bond lengths in the silicon complexes was found to change with increasing amount of the potas­sium ions. It was established that the following complexes [SiO₄]^4-, [SiO₃F]^3- and [SiF₆]^2- are the most stable in 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 ₃SiF₇, is unstable in this tempera­ture range. In the KF−KCl−K₂SiF₆−SiO₂ melts, 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 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 in situ data of Raman spectroscopy for the KF−KCl−K₂SiF₆ and KF–KCl–K₂SiF₆–SiO₂ melts.