Structural Plasticity in Intermetallic Compounds: Interpreting Complexity as a Structural Response to Chemical Pressure

Several parameters are known to influence the structural chemistry of intermetallic compounds, but our overall understanding of the forces that shape these structures is incommensurately poor relative to the extensive array of structure types we know to exist. Close examination of the diversity of intermetallic compounds reveals a possible foothold in the overwhelming variety: large, complex unit cells often have close relationships to compounds with simpler structures. Complex compounds can often be broken down into the sum of fragments of more easily understood structures, plus any new coordination occurring at fragment interfaces. Beginning to organize compounds into structural progressions inspired the hypothesis of structural plasticity, the centerpiece of this work: The possibility that a complex, fragmented structure arises in response to the stress inherent in otherwise simple materials. The source of this stress, proposed to be intrinsic to the compound, is chemical pressure; a strain created in a structure when a stoichiometry incompatible with a simple geometry experiences mismatch in atomic size or electron count. The rearrangement of atoms occurring in response to this strain results in a different compound, related to the first by structural plasticity. We obtain our nomenclature from a similar phenomenon seen in elemental metals: on application of external stress, dislocations or defects appear in a simple metallic lattice to alleviate strain. On a structural level in intermetallic compounds, plasticity manifests itself in relation to the stress of suboptimal local interactions. We examine this computationally using Density Functional Theory-based chemical pressure analysis (DFT-CP). This document outlines the elucidation of the idea of structural plasticity, beginning with experimentally synthesized compounds whose unit cells exhibit fragmented versions of related phases. After synthesizing several new compounds, the stage of development of the CP tools available inspired advancement of new chemical pressure methods. We show examples of structural plasticity along with the development of DFT-CP. To conclude, we illustrate the structural plasticity of the CaCu5-type in the form of a family of related compounds: Ca2Ag7, Ca14Cd51, and CaCd6, a Tsai-type quasicrystalline approximant. The diversity of all these phases can be traced back to structural adaptations in response to chemical pressure.