A Cavalcade Of Clusters: The Interplay Between Atomic And Electronic Structure In Complex Intermetallics

Abstract

What drives the stability of complex intermetallic compounds? Many, if not most, metals and alloys crystallize in one of the familiar body-centered cubic (bcc), face-centered cubic (fcc), or hexagonal close-packed (hcp) crystal structures. But what of Cu5 Zn8 with 54 atoms in its unit cell, or Ti2 Ni with its 96? Or larger still Fe11 Zn39 , with 408 atoms in the repeat unit? Chemically and physically straight-forward semi-empirical calculations can ¨ lend us insight, and we review one such technique-the extended Huckel method-with new examples. Along the way, we solve the crystal structures for the new family of ternary compounds Ir x Ru1[-] x Zn10 and see what clues the Ir site preferences can give us about the connection between atomic structure and electronic structure via the Mott-Jones method of constructing electronic wavefunctions from the intensity of Bragg peaks. Finally, we investigate the connection between intermetallic structures that possess pseudo five-fold diffraction patterns. We find these peaks corresponding to features of several different types of tetrahedral close-packing, which in turn can all be described as three-dimensional projections of a four-dimensional perfect packing of tetrahedra: the 600-cell. We further show that the Bragg peaks comprising the pseudo five-fold diffraction pattern are linked to structural stability, and hence the higher-dimensional geometry.