ISSN:
1600-5740
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
Ammonioguanidinium hexafluorosilicate, CH8N_4^{2+}·SiF_6^{2-}, and bis(aminoguanidinium) hexafluorosilicate dihydrate, 2CH7N_4^+·SiF_6^{2-}·2H2O, are new materials formed as by-products in course of preparing ferroelectric CH8N4ZrF6 in the presence of glassware. Their structures were determined for comparison with the corresponding hexafluorozirconates. All atoms including the eight H atoms in the CH8N_4^{2+} cation and the seven H atoms in the CH7N_4^+ cation have been located and refined with wR(F2) = 0.0653, R = 0.0255, S = 1.146 and wR(F2) = 0.0745, R = 0.0301, S = 1.065, respectively. The N2C—N—N backbone of the 2+ cation is close to planarity, while that of the 1+ cation does not differ significantly from planarity. The SiF_6^{2-} octahedron is nearly ideally regular in both materials, with 〈Si—F〉 = 1.684 (unbiassed estimator of standard uncertainty = 0.016) Å in the anhydrous hexafluorosilicate and 1.6801 (unbiassed estimator of standard uncertainty = 0.0006) Å in the dihydrate. The combination of coulombic and NH...F interactions in CH8N4SiF6 results in a relatively dense variant of the NaCl structure. In addition to similar forces, the dihydrate is also characterized by the role of the water molecule with its strong NH...O interactions; its packing efficiency is, however, appreciably less than that of the anhydrous hexafluorosilicate with an ∼8% increase in void space. Cleaved crystals of the dihydrate are frequently twinned across the (001) composition plane, with a twofold rotation about the b axis as the twin operation.The systematic names of the compounds studied in this paper are given following IUPAC recommendations. The corresponding fluorozirconates have previously been referred to in the literature by the non-IUPAC names aminoguanidinium(2+) hexafluorozirconate and diaminoguanidinium(1+) hexafluorozirconate for ammonioguanidinium(2+) hexafluorozirconate and bis[aminoguanidinium(1+)] hexafluorozirconate, respectively.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0108768198013044
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