Infrared spectra of the ammonium ion in crystals. Part XIII. Crystal structure of (NH4)2[AlF5(H2O)] and NH3D+ probe-ion spectra in (NH4)2[AlF5(H2O)], NH4AlF4, and (NH4)3ZnCl5, with remarks on structural filiation of AMF4 fluorides

Abstract

The crystal structure of (NH4)2AlF5•H2O = (NH4)2[AlF5(H2O)] (Pbcn, a = 10.192(4) Å, b = 8.037(2) Å, c = 7.844(1) Å, Z = 4) consists of isolated [AlF5(H2O)]2− octahedral and NH4+ ions. The octahedra are linked by [Formula: see text] bonds to form zigzag chains parallel to c and the chains are cross-linked by normal [Formula: see text] hydrogen bonds to the NH4+ ions. The ir characteristics of the hydrogen bonds in this and the other two title compounds were probed with the NH3D+ ion between 10 and 293 K. The probe-ion spectra confirm the C1 symmetry of the ammonium ion in (NH4)2[AlF5(H2O)] and point to [Formula: see text] bonding of moderate strength. For NH4AlF4 the spectra agree with the expectation from the known crystal structure of both the ordered and the disordered phase, but the transition at ~150 K is not evident in the evolution of the spectra with temperature. Detailed assignment of the ND stretching and bending component absorptions of NH3D+ is not possible for (NH4)3ZnCl5 = (NH4)3(ZnCl4)Cl. However, the unusually low frequencies of two of the components of the ND stretching absorptions in this crystal indicate the existence of [Formula: see text] bonds stronger than those in NH4Cl. The filiation of the known AMF4 structures deriving from TlAlF4 is presented in terms of two-colour space groups.