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The Crystalline Hydrates of Hexafluorosilicic Acid: A Combined Phase‐Analytical and Structural Study
Author(s) -
Mootz D.,
Oellers E.J.
Publication year - 1988
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.19885590103
Subject(s) - hydrogen bond , hydrate , oxonium ion , diffractometer , crystallography , phase diagram , chemistry , thermal stability , crystal structure , ionic bonding , cationic polymerization , powder diffraction , atmospheric temperature range , phase (matter) , analytical chemistry (journal) , ion , molecule , thermodynamics , organic chemistry , physics
The system hexafluorosilicic acid‐water was studied by low‐temperature difference thermal analysis and X‐ray powder diffraction in the water‐rich range of 80‐‐100 mol% H 2 O. A quasi‐binary behavior was found and the melting diagram constructed. It shows the existence and stability ranges of three crystalline hydrates H 2 SiF 6 · nH 2 O with n = 4, 6, and 9.5. They melt congruently at 20 and −12°C, and incongruently at −54°C, respectively. The hydrates were further characterized by determination of their structures from single‐crystal MoKα diffractometer data. They were found to be oxonium salts. The ionic formulae, in the order of increasing water content, are (H 5 O 2 ) 2 SiF 6 , (H 5 O 2 ) 2 SiF 6 · 2 H 2 O, and (H 5 O 2 )(H 7 O 3 )SiF 6 · 4.5 H 2 O. The structures are governed by extensive OH ⃛O and OH ⃛F hydrogen bonding. The water structure of the 9.5‐hydrate, with the cationic and neutral species taken together, is an unusual three‐dimensional network which hydrogen‐bonds the anions in channels.