The Story Behind Disorder: Determining the Relationship Between Structure and Physical Properties of Disordered Intermetallics

The synthesis and characterization of single crystalline ternary intermetallics is the primary focus of this dissertation. By focusing on specific structural subunits, we can design materials with tailorable magnetic and electrical properties. Our rationale for the pursuit and design of low dimensional intermetallics is not unlike the assembly of hierarchical structures. We endeavor to grow these materials as bulk high quality single crystals thereby granting us the assurance that intrinsic properties are determined. We grow single crystals primarily by utilizing the flux growth method, which allows the growth of congruently melting and metastable phases. Highlighted in this dissertation are the synthesis, structures, and physical properties of LnMₓGa₃ (Ln = Ho, Er; M = Fe, Co; x < 0.2) of the AuCu₃ type, the La₆MnSb₁₅ structure type, and compounds adopting the La₂Fe₄Sb₅ structure type. Special emphasis is placed on the investigation of the effects of disorder and substitution on emergent physical phenomena. In the case of LnMₓGa₃ (Ln = Ho, Er; M = Fe, Co; x < 0.2), we were motivated to study the insertion of transition metals into compounds adopting the AuCu₃ structure type. Although HoGa₃ has five allotropes, a distorted AuCu₃-type structure is stabilized upon addition of transition metal. The HoMₓGa₃ order antiferromagnetically and the Néel temperatures are influenced by Ho-Ho contact distances. The incorporation of transition metals into the body position of the ErGa₃ host structure resulted in strengthened rare earth coupling as evidenced by higher transition temperatures that what is observed in the parent ErGa₃ compound. We then turned our attention to the La₆MnSb₁₅ structure type, which is comprised of anionic antimony nets and triangular units of rare earth ions. We found that the Mn analogues, La₆MnSb₁₅ (Ln = La, Ce) exhibit long-range occupational disorder that can be modeled using a supercell structure. The Zn-containing compounds Ln₆Mn₁₋ₓZnₓSb₁₅ (x ~ 0.5) and Ln₆ZnSb₁₅ (Ln = La-Pr) were found to be highly disordered but did not exhibit long-range occupational disorder. In our search for highly correlated low-dimensional materials, we posited the idea of “building up” low-dimensional structures by beginning with layered materials and envisioning the incorporation of transition metal sublattices in the final structure. The La₂Fe₄Sb₅ structure type can be thought of as building up of a SmSb2 structure type and a transition metal sublattice with nearly equilateral triangles of iron and the potential for geometric frustration. The rare earth and transition metal sublattices both carry a magnetic moment and exhibit localized magnetism. The synthesis, characterization, and interplay of the f- and d-electrons and the mechanisms of localized and itinerant behavior as a function of Co concentration in La₂Fe₄₋ₓCoₓSb₄․₈Bi₀․₂ (x ~ 0 and 0.5) and Ce₂Fe₄₋ₓCoₓSb₄․₈Bi₀․₂ (x ~ 0.5 and 1) is discussed.