A quantum mechanically guided view of Mg44Rh7

We present a new geometric description of Mg44Rh7, a compound with 408 atoms in its cubic unit cell. Using both experimental site preferences and LDA-DFT-calibrated extended Huckel (eH) calculations as guides, we highlight the structural units within Mg44Rh7 that reflect the electron-richness or electron-poorness of each crystallographic site. The units that best account for these site preferences and electron populations are 34- and 25-atom fragments of the Ti2Ni structure, rather than the variety of clusters often used to describe complicated intermetallic and ionic structures. These Ti2Ni pieces, located using a systematic search algorithm, fit together in a beautifully intricate network. An examination of this network reveals some surprising geometric features of Mg44Rh7, including a fractal-like arrangement of similar atomic formations on different length scales, geometrically connected to an approximate fivefold symmetry.