Author: Hillary Mitchell Mitchell Warden
Publisher:
Published: 2020
Total Pages: 0
ISBN-13:
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Intermetallic compounds adopt structures that run the gamut of complexity, from simple packings of spheres to structures that require more than three dimensions to fully describe. These diverse intermetallic structures are all influenced by simple chemical factors, such as atomic radius ratios and valence electron concentration, but their role is often poorly understood, hindering the rational design of new intermetallics with potentially useful properties. Previous work in the Fredrickson Group has found that complex structural arrangements can arise from tensions in electron count or atomic packing in a simple parent structure (via the reversed approximation Molecular Orbital and the DFT-Chemical Pressure methods, respectively). Consideration of a single chemical factor, then, enables us to explain why a complex structure forms after synthesis and crystallographic characterization, but it does not provide enough specificity to predict the structural deviation. Making predictions about complex intermetallic structures requires knowing which of several paths a parent structure will take to relieve these issues. In this thesis, we will show that concerted investigation of both atomic size and electron count allows us to navigate a structure's potential configurations and guide our expectations for experimentally observed structures. This concept-called the Frustrated and Allowed Structural Transitions (FAST) principle-says that an allowed structural transition is favorable with respect to both factors, while a frustrated transition has a favorable path in one factor and a constrained path in the other. In each chapter, we present the FAST analysis of the tensions within a parent intermetallic structure that explain and even anticipate the observed experimental structural chemistry. We begin in Chapter 2 with the polymorphism of PtGa2, in which the electronics drive the formation of a complex tetragonal superstructure of the fluorite type while atomic packing enables the stabilization of the electronicallydisfavored cubic form via the incorporation of Pt interstitials. Next, in Chapter 3, we use the FAST analysis in a more predictive way to anticipate complexity in IrSi, which proves to be an incommensurately modulated perturbation of the MnP type. The major form of the modulations (Ir-Ir bonding along directions of negative CP) solves the electronic issues of the MnP type, while the tendency of Ir bonds to form as trimers arises from atomic packing issues between the Ir and Si sublattices. Chapter 4 focuses on the temperature-dependent structural variations of PdBi, where the room temperature polymorph is driven to form by the electron counting of the phase and the high temperature, incommensurate polymorph is favored by entropy and vibrational freedom seen in the parent structure's CP scheme. Finally, in Chapter 5, we use the FAST principle to investigate the CP-driven, electronically-guided structural transition away from the BaAl4 type leading to the incommensurately modulated compound, Dy(Cu0.18Ga0.82)3.71(1). In this case, the modulations lead to block-like incorporation of dumbbell/single atom substitutions that shrink the too-large coordination environments around the Dy atoms, while also balancing the number of substitutions and the amount of Cu incorporated, as both effects lower the electron count of the phase. The work of this thesis establishes the FAST principle as a guideline for understanding complex structural chemistry in intermetallics, laying the groundwork for a predictive chemical framework. As the FAST principle is further refined, we hope to incorporate more chemical factors, such as electronegativity, into the analysis, as well as develop rules for how CP schemes dictate structural chemistry.
