X-Ray, Raman, infrared, and nuclear magnetic resonance studies of the crystal structure of ammonium tetrachloroaluminate, NH4AlCl4

Abstract

Ammonium tetrachloroaluminate crystallizes in the orthorhombic System a = 11.022(6) Å, b = 7.072(3) Å, c = 9.257(5) Å, Z = 4, space group Pnma. The structure was determined by direct methods from diffractometer data and refined by full-matrix least-squares procedures to final R and R_w values of 0.053 and 0.069 respectively, for 596 reflections with I > 2σ(I). Hydrogen atoms were not observed. Al—Cl bond lengths are similar to those of other alkaline chloroaluminate salts (Li⁺, Na⁺, NO⁺). The salt is isostructural with NH₄ClO₄.Crystals and polycrystalline samples of this compound have been studied by Raman scattering above 10 K, and infrared spectra of specimens of a polycrystalline sample have been recorded in Nujol and a fluorolube above 80 K. Both methods enabled us to observe the bands corresponding to the ν₁(A₂), ν₃, and ν₄(F₂) vibration modes of the ammonium ion but the infrared spectra contain no band due to combination modes involving the librations of this ion. Nuclear magnetic second moment remains equal to 4 G² over the range 298–80 K. These spectroscopic results, compared to those of other ammonium salts, indicate that the ammonium ion seems to be freely rotating.The four characteristic bands of the AlCl₄⁻ tetrahedral anion have been studied by Raman scattering in the 298–10 K temperature range.