Investigation into the structural chemistry of Cu2+ ions in doped nearly perfect single crystals of ammonium sulfate with reference to their role in habit modification

Abstract

A unique combination of experimental and theoretical techniques is used in a self-consistent investigation into the habit modification of nearly perfect single crystals of ammonium sulfate by trace amounts of Cu²⁺ ions. X-Ray absorption spectroscopy studies reveal that the dopant ions remain in their divalent state following crystallisation and are uniformly distributed throughout the crystal. Analysis of EXAFS spectra of the doped crystals is consistent with the Cu²⁺ ion being linked to four water molecules in a square-planar equatorial arrangement with weaker interactions with axial oxygen atoms from two of the host sulfate groups. This existence of a hydrated copper complex within the habit-modified ammonium sulfate is also confirmed using UV–VIS spectroscopy. X-Ray standing-wave spectroscopic data of the Cu²⁺-doped crystals in comparison with crystals spectroscopically labelled with Rb⁺ ions are consistent with the Cu²⁺ substituting for the SO^2–₄ ion, thus involving a substantial structural rearrangement with the associated redistribution of ionic charge. The data are rationalised with the aid of ab initio quantum chemistry calculations which reveal that the charges on the hydration protons closely correlate with those on the ammonium ions in ammonium sulfate. This similarity allows the charged water molecules to mimic the ammonium ions during adsorption, resulting in a complex substitution of an SO^2–₄ ion by the hydrated Cu²⁺ complex associated with the removal of four NH⁺₄ ions to maintain strict charge neutrality. The overall ramifications of the incorporated ions' structure on the resultant habit modification are discussed and rationalised in terms of morphological modelling theory.