Anhydrous ammonioguanidinium(2+) and dihydrated bis[aminoguanidinium(1+)] hexafluorosilicates: new co-products of preparing ferroelectric ammonioguanidinium(2+) hexafluorozirconate

Abstract

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(F 2) = 0.0653, R = 0.0255, S = 1.146 and wR(F 2) = 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.