Mg₁₁Cu₆Al₁₂, a new link in the structural chemistry of MgCu₂-type clusters

We report the synthesis and crystal structure of a new phase in the Mg-Cu-Al system: Mg₁₁Cu₆Al₁₂. This compound crystallizes in the K₁₇In₄₁ structure type. When written asMg_17-xCu_xAl₁₂, x = 6, the composition of this phase foretells a connection to Mg₁₇Al₁₂ (α-Mn type). The structures of both can be constructed from 29-atom fragments of the MgCu₂ structure type. They differ in the orientations of these fragments: the Mg₁₁Cu₆Al₁₂ structure is obtained when half of the MgCu₂-type clusters of Mg ₁₇Al₁₂ are rotated by 90°. Electronic structure calculations using density functional theory (DFT) and the extended Hückel (eH) method point to driving forces for this structural transformation. Density of states (DOS) curves calculated for Mg₁₁Cu₆Al ₁₂ in the two structure types indicate that both are stabilized by DOS minima close to the Fermi energy, with the pseudogap being deeper for the observed structure. An eH relative Mulliken population analysis reveals that cluster rotation also changes the electronic character of the outermost sites of the MgCu₂-type fragments: six atoms per formula unit go from being ambiguous to anionic, and thus suitable to occupation by relatively electronegative elements. These are the positions occupied by Cu in Mg ₁₁Cu₆Al₁₂. The creation of these six anionic sites adapted to occupation by Cu provides an impetus for cluster rotation. The removal of ambiguity of the outermost sites of the MgCu₂-type fragments creates extended networks of anionic and cationic sites in Mg ₁₁Cu₆Al₁₂. The cationic sites trace out the clathrate II framework noted by Corbett and co-workers in the K ₁₇In₄₁ type, while the anionic sites ensheathe this framework to complete Friauf polyhedra around the framework atoms.