Crystal structure of CaSiF6·2H2O(mP2) and reevaluation of the SiIV–F bond-valence parameter R 0

Abstract

The structure of a second polymorph of CaSiF6·2H2O [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[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...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 SiIV–F bond-valence parameter R 0. Based on a data set of 42 high-quality crystal structures containing 49 independent SiIV coordination environments, a revised value of 1.534 Å was derived for R 0.