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Mg 11 Cu 6 Al 12 , A New Link in the Structural Chemistry of MgCu 2 ‐Type Clusters
Author(s) -
Berns Veronica M.,
Stacey Timothy E.,
Sapiro Michael,
Fredrickson Daniel C.
Publication year - 2011
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201100335
Subject(s) - chemistry , mulliken population analysis , crystallography , cluster (spacecraft) , density functional theory , atom (system on chip) , crystal structure , type (biology) , pseudogap , electronic structure , fermi level , population , computational chemistry , condensed matter physics , physics , doping , quantum mechanics , ecology , cuprate , demography , sociology , computer science , embedded system , biology , programming language , electron
We report the synthesis and crystal structure of a new phase in the Mg‐Cu‐Al system: Mg 11 Cu 6 Al 12 . This compound crystallizes in the K 17 In 41 structure type. When written asMg 17– x Cu x Al 12 , x = 6, the composition of this phase foretells a connection to Mg 17 Al 12 (α‐Mn type). The structures of both can be constructed from 29‐atom fragments of the MgCu 2 structure type. They differ in the orientations of these fragments: the Mg 11 Cu 6 Al 12 structure is obtained when half of the MgCu 2 ‐type clusters of Mg 17 Al 12 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 Mg 11 Cu 6 Al 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 MgCu 2 ‐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 11 Cu 6 Al 12 . 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 MgCu 2 ‐type fragments creates extended networks of anionic and cationic sites in Mg 11 Cu 6 Al 12 . The cationic sites trace out the clathrate II framework noted by Corbett and co‐workers in the K 17 In 41 type, while the anionic sites ensheathe this framework to complete Friauf polyhedra around the framework atoms.