Crystal structure of CaSiF 6 ·2H 2 O( mP 2) and reevaluation of the Si IV -F bond-valence parameter R 0 .

The structure of a second polymorph of CaSiF 6 ·2H 2 O [calcium hexafluorido-silicate dihydrate; space group P 2/ c (No. 13), Pearson symbol mP 2] 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(H 2 O) 2/1 (SiF 6 ) 4/4 ] layers oriented parallel to the bc -crystallographic plane, a unique motif among structurally characterized hydrated hexa-fluorido-silicates. The crystal structure also exhibits inter- and intra-layer hydrogen bonds, with the inter-layer 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 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 Å was derived for R 0 .