Intermetallic phases comprise a large class of solid state compounds which is remarkable for its diverse structural chemistry as for the valuable physical and chemical properties it offers.  Intermetallic crystal structures range from simple variants of elemental structure types to quasicrystals whose structures are of such complexity that they defy description using 3-dimensional unit cells.    In this presentation, we will discuss how an iterative approach of experiment and theory can reveal bonding factors familiar from molecular chemistry underlying the crystal structures of these inorganic materials.  Our focus will be on the local clusters of the three families of icosahedral quasicrystals (Mackay, Bergman, Tsai), each constructed from different sequence of concentric icosahedral shells. From these three families, three distinct origins for local icosahedral order will emerge:  (1) the chemical pressure-driven contraction of hexagonal to pentagonal coordination polyhedra in the Tsai-type clusters of CaCd6, (2) a micelle-like phase segregation between Ca-Cd and Cu-Cd interactions in the Bergman-type clusters of our new phase Ca10Cu2Cd27, and (3) the optimization of acid-base neutralization in the Mackay-type clusters of Sc44Ir7 and Sc57Ir13.  These three examples illustrate how the beautiful structural motifs encountered in intermetallics can be understood as the result of chemical interactions, often in ways suggestive of new synthetic endeavors and paths toward the ultimate goals of structural control and materials design.

Diverse chemical origins of local icosahedral order in quasicrystals: How multi-shelled clusters arise in intermetallics from micelle-like segregation, chemical pressure relief, and structural acid-base chemistry

248th National Meeting (2014)

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