Description:
<jats:title>Abstract</jats:title><jats:p>We report the synthesis and crystal structure of a new phase in the Mg‐Cu‐Al system: Mg<jats:sub>11</jats:sub>Cu<jats:sub>6</jats:sub>Al<jats:sub>12</jats:sub>. This compound crystallizes in the K<jats:sub>17</jats:sub>In<jats:sub>41</jats:sub> structure type. When written asMg<jats:sub>17–<jats:italic>x</jats:italic></jats:sub>Cu<jats:italic><jats:sub>x</jats:sub></jats:italic>Al<jats:sub>12</jats:sub>, <jats:italic>x</jats:italic> = 6, the composition of this phase foretells a connection to Mg<jats:sub>17</jats:sub>Al<jats:sub>12</jats:sub> (α‐Mn type). The structures of both can be constructed from 29‐atom fragments of the MgCu<jats:sub>2</jats:sub> structure type. They differ in the orientations of these fragments: the Mg<jats:sub>11</jats:sub>Cu<jats:sub>6</jats:sub>Al<jats:sub>12</jats:sub> structure is obtained when half of the MgCu<jats:sub>2</jats:sub>‐type clusters of Mg<jats:sub>17</jats:sub>Al<jats:sub>12</jats:sub> 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<jats:sub>11</jats:sub>Cu<jats:sub>6</jats:sub>Al<jats:sub>12</jats:sub> 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<jats:sub>2</jats:sub>‐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<jats:sub>11</jats:sub>Cu<jats:sub>6</jats:sub>Al<jats:sub>12</jats:sub>. 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<jats:sub>2</jats:sub>‐type fragments creates extended networks of anionic and cationic sites in Mg<jats:sub>11</jats:sub>Cu<jats:sub>6</jats:sub>Al<jats:sub>12</jats:sub>. The cationic sites trace out the clathrate II framework noted by Corbett and co‐workers in the K<jats:sub>17</jats:sub>In<jats:sub>41</jats:sub> type, while the anionic sites ensheathe this framework to complete Friauf polyhedra around the framework atoms.</jats:p>