Crystal structure of CaSiF₆?2H₂O(mP2) and reevaluation of the Si^IV-F bond-valence parameter R₀

The structure of a second polymorph of CaSiF6 center dot 2H(2)O [calcium hexafluoridosilicate dihydrate; space group P2/c (No. 13), Pearson symbol mP2] was elucidated by single-crystal X-ray diffraction. It arose as an unexpected product when soda-lime glass was attacked by HF. Its crystal structure consists of infinite (2)(infinity)[Ca(H2O)(2/1)(SiF6)(4/4)] layers oriented parallel to the bc-crystallographic plane, a unique motif among structurally characterized hydrated hexafluoridosilicates. The crystal structure also exhibits inter- and intralayer hydrogen bonds, with the interlayer O-H center dot center dot center dot O hydrogen bonds involving a disordered hydrogen atom. The large deviation between the calculated bond-valence sum for Si and the expected value prompted a redetermination of the empirical Si-IV-F bond-valence parameter R-0. Based on a data set of 42 high-quality crystal structures containing 49 independent Si-IV coordination environments, a revised value of 1.534 angstrom was derived for R-0.