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

The length and energy of bonds in the complex anions of silicon formed in KF−KCl−K 2 SiF 6  and KF−KCl−K 2 SiF 6 –SiO 2  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 [SiO 4 ] 4- , [SiO 3 F] 3-  and [SiF 6 ] 2-  are the most stable in KF−KCl−K 2 SiF 6  and KF−KCl–K 2 SiF 6 –SiO 2  melts. The [SiO 4 ] 4-  and [SiF 6 ] 2-  complexes are thermally stable in the molten salt in the temperature range of 923–1073 K, whereas the [SiF 7 ] 3-  structure, which is typical for the lattice of crystalline 3 SiF 7 , is unstable in this temperature range. In the KF−KCl−K 2 SiF 6 −SiO 2  melts, conditions above 1043 K were created allowing the transformation of [SiО 3 F] 3-  into [SiO 4 ] 4- . Within the studied temperature mode, the Si–F bond length is in the range 1.5–1.9 A and the Si–O bond lengths is 1.5–1.7 A. The obtained results are in a good agreement with in situ data of Raman spectroscopy for the KF−KCl−K 2 SiF 6  and KF–KCl–K 2 SiF 6 –SiO 2  melts.