One-dimensional infinite chain structures of [Al2(OH)4(H2O)4]X2 (X = I, Br, Cl): an aggregate of Al2 species and a precursor of Al(OH)3

Abstract

By a combination of the Rietveld full-profile fitting technique based on powder X-ray diffraction data and the single-crystal structure solving and refining method, one-dimensional infinite zigzag chain structures were observed in the structures of [Al₂(OH)₄(H₂O)₄]X₂ (X = I, Br, Cl). These crystallize in the same monoclinic system and the same C2/c space group with different unit cell parameters from spontaneously hydrolyzed solutions of AlX₃. Each chain is composed of a significant number of AlO₆ octahedra that fall into two groups with reverse orientations and connect to each other by edge-sharing. Furthermore, the counterions intersperse among the chains forming strong van der Waals interactions. It was also discovered that each chain was an aggregate formed from the further hydrolysis of the Al₂ species (Al₂(OH)₂(H₂O)₈⁴⁺, a dimer formed by two octahedral Al(H₂O)₆³⁺ monomers sharing an edge) and a building unit for constructing the infinite hexameric ring sheet in nordstrandite and gibbsite. These are three new structures, rarely solved from powder XRD data, of polyaluminum compounds, and they provide the first direct evidence of the aggregation processes of Al₂ species and their subsequent evolution into an infinite zigzag chain as well as further evolution into an infinite hexameric ring layer as found in nordstrandite and gibbsite. Furthermore, they also represent the first three examples in which each polyaluminum species possesses a one-dimensional infinite chain structure formed by AlO₆ octahedra via edge-sharing only.