Structural And Electronic Investigations Of Complex Intermetallic Compounds PDF Download

Intermetallic compounds-a class of material comprised only of metallic elements-encompass an enormous range of structural chemistry and diversity. The simplest of these structures can be described with only a few atoms or as variations on simple packings, but others transcend explanations in three dimensions and comprised thousands of atoms. Describing and rationalizing the formation of this immensely complex structures requires connections between them and simpler parent structures. These connections then enable the use of fundamental chemical factors-such as atomic size (studied through the DFT-Chemical Pressure Method), electron counts (studied through the 188́2n electron counting rule), and electronegativity (studied through Bader charge transfer)-to investigate the destabilizing factors of the parent structure driving the formation of the more complex. A framework that embraces these fundamental chemical ideas is the Frustrated and Allowed Structural Transitions (FAST) principle. Through the FAST framework, chemical factors can be weighed as either favoring or disfavoring a structural transition, and multiple factors can then be added to discern the allowed or frustrated nature of the transition. Such an analysis creates a foundation for both the rationalization and prediction of structural transformations. This thesis will show how comprehensive investigations into fundamental chemical factors are used to explain and predict complex structural behavior in intermetallics. The first part of this thesis (Chapters 1 - 3) focuses on the descriptive side of the FAST model. Beginning with Chapter 1, fcc stacking variants ZrAl3 and TiAl3 -though isoelectronic-form different structure types, the former type electronically precise, the latter, electronically poor per the 188́2n rule. Strains within the Al sublattice as a result of the too-small size of Ti inhibits the bond formation necessary for electronically precise phase. Adding electrons into the TiAl3 electron poor structure type in the synthetic phase ZrAl3-xSnx (x ~ 0.4) relieves both electronic and atomic packing strains in an allowed structural transition. In Chapter 2, structural responses to atomic packing strains are explored through the concept of structural plasticity in the Mo-Fe-Cr system. Here, stresses in the MgZn2-type MoFe2 parent phase reveal CP strains enabling layer insertion towards the formation of the Mo6Fe7 phase. Subsequently, the homogeneity range of the layered phase are explored through a dual lens of atomic size and electronegativity. The strains in the parent structure are further used to elucidate driving forces for the formation of the Mo5Fe24 phase, and Cr substitution is rationalized through atomic size and electronegativity requirements. Chapter 3 reinvestigates the inhomogeneous packing of the EuMg5-type YZn5, revealing a near continuous column of electron density assigned to a complex occupation pattern of Zn atoms resulting in the YZn5.225 phase. The Zn occupation pattern is attributed to communication of atomic packing strains within the columnal motifs, paving the way for superstructure formation in an allowed transition. The predict half of this Thesis uses atomic packing strains to highlight CP features called quadrupoles, which through connections with phonon modes, allow the prediction of soft atomic motions. Chapter 4 defines a metric quantifying a CP quadrupole and explores its prevalence in the TE2 (T = transition metal, E = main group element) library of structures. Four systems with highly quadrupolar sites exhibiting structural phenomena such as superstructure formation, polymorphism, atomic diffusing, and Li intercalation are investigated as to the paths of soft atomic motion enabling thse phenomena. The predictive nature of quadrupoles are further applied in Chapter 5 to a series of RE-Ru (RE = Y, La, Sc) and Ca-Sn superstructures sharing the same parent structure. Finally, Chapter 6 is an overview of the research ideas and methods of the Fredrickson group written in a style readable for the general audience. Throughout these Chapters encompassing both synthetic and theoretical investigations, the FAST model will be utilized to rationalize existent structural transitions and used predictively to anticipate complex structural phenomena.