Acta Crystallographica Section B

Structural Science

Volume 55, Part 2 (April 1999)

research papers

Acta Cryst. (1999). B55, 246-254 [ doi:10.1107/S0108768198013044 ]

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

C. R. Ross II, M. R. Bauer, R. M. Nielson and S. C. Abrahams

Abstract: Ammonioguanidinium hexafluorosilicate, CH₈N $_4^{2+}$ ·SiF $_6^{2-}$ , and bis(aminoguanidinium) hexafluorosilicate dihydrate, 2CH₇N·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 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 NHF 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 NHO 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.