Zintl phases have been the focus of recent thermoelectric research due to their complex crystal structures, which include covalently bonded anionic sub-structures in a lattice of electropositive cations. The covalent bonds lead to high mobility, while strict electron-counting rules contribute to the formation of complex structures, which in turn lead to low thermal conductivity. In this manner,these compounds can fit the ideal phonon-glass and electron-crystal model for thermoelectric materials. Although Zintl phases are a promising class of thermoelectric materials that have been studied intensively since 2005, there are still several important fundamental questions that remain unanswered. These include questions related to anisotropic transport and how it relates to the crystalstructure, and the role played by intrinsic defects in determining carrier concentration. Additionally, the field of Zintl compounds is ever expanding; through the use of exploratory single crystal growths and the careful selection of starting composition, novel compounds and structure types can be discovered that may be promising thermoelectric candidates.Zintls with the Ca5M2Sb6 (M = Al, Ga, In) structure type, characterized by one-dimensional, ladder-like polyanions, were previously predicted to have highly anisotropic electrical conductivity. To investigate this anisotropic behavior, single crystals of Ca5M2Sb6 (M = Al, Ga, In) were grown in the current work via the self-flux method. These crystals grew preferentially along the polyanionic "ladders" of the structure, but only measured a few millimeters long by tens of microns thick. Characterizing the transport properties of these crystals both parallel and perpendicular to the growth direction demanded a novel characterization technique, as placing contacts by hand wasinfeasible in the perpendicular direction. Micro-fabrication techniques will be utilized whereby micro-ribbons are extracted from crystals both perpendicular and parallel to the preferred growth direction using a focused ion beam milling technique. Photolithography was then utilized to create a circuit of sensors for transport measurements. The resistivity, carrier concentration, and mobility of a micro-ribbon of Ca5In2Sb6 perpendicular to the preferred growth direction was successfully characterized using this approach. Resistivity measured in the parallel direction using a four-probe resistivity setup was found to be nearly 20 times higher than the perpendicular direction, confirming theoretical predictions.Experimental investigation of intrinsic defects in single crystals is also explored in the promising Mg3Sb2 system, accomplished using single crystal X-ray diffraction. The defect chemistry of this system for both Mg- and Sb-rich single crystal synthesis is investigated, where vacancies and interstitial sites are identified and quantified in collaboration with researchers at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany.Lastly, the discovery of a new quaternary Zintl phase, Ca9Zn3.1In0.9Sb9 is reported, which was discovered as a by-product during the attempted growth of Zn-doped Ca5In2Sb6. The new Ca9Zn3.1In0.9Sb9 structure was solved with the help of collaborators at the University of Delaware. Measurements of the electrical resistivity of the Ca9Zn3.1In0.9Sb9 crystals performed at MichiganState University showed results similar to that of already-optimized Ca9Zn4.5Sb9 compounds, pointing to promising thermoelectric performance.
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