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 (NH 4 ) 2 AlF 5 •H 2 O = (NH 4 ) 2 [AlF 5 (H 2 O)] (Pbcn, a = 10.192(4) Å, b = 8.037(2) Å, c = 7.844(1) Å, Z = 4) consists of isolated [AlF 5 (H 2 O)] 2− octahedral and NH 4 + 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 NH 4 + ions. The ir characteristics of the hydrogen bonds in this and the other two title compounds were probed with the NH 3 D + ion between 10 and 293 K. The probe-ion spectra confirm the C 1 symmetry of the ammonium ion in (NH 4 ) 2 [AlF 5 (H 2 O)] and point to [Formula: see text] bonding of moderate strength. For NH 4 AlF 4 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 NH 3 D + is not possible for (NH 4 ) 3 ZnCl 5 = (NH 4 ) 3 (ZnCl 4 )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 NH 4 Cl. The filiation of the known AMF 4 structures deriving from TlAlF 4 is presented in terms of two-colour space groups.