UC Davis

Zintl phases are an extremely broad class of compounds that are charge balanced semiconductors. They follow simple assumptions where the more electropositive atoms donate their electrons (cations) to the more electronegative atoms (anions) which may or may not form additional bonding networks to satisfy their valence. Given the large scope of this category, this naturally brings about a huge number of possible structures and range of physical phenomenon. The understanding of how the atomic structure relates to the physical properties is fundamental in the design and optimization of new materials. Thermoelectric materials are a unique type of material that can convert heat directly into electricity in a reversible fashion. These materials can be used in power generators to recycle and convert waste heat into usable electricity. Given that there are no moving parts in these thermoelectric generators and are only based on the solid-state materials, they are an extremely reliable source of power and commonly used on long term space missions where other power sources are not realistic or robust enough. Their widespread application is however, currently being held back by their conversion efficiencies. The efficiency of a thermoelectric material is dependent on zT=(α^2 T)/ρκ , the figure of merit, where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. To maximize zT, a compound would need to have low electrical resistivity, low thermal conductivity and a high Seebeck coefficient. The optimization of these parameters is non-trivial due to their interconnected nature with carrier concentration. Optimization schemes such as a single parabolic band model can provide directions to probe experimentally. The viability of this method will be visited in this work. The first part of this dissertation will focus on the structure and properties of the Yb21Mn4Sb18 phase. This material was predicted to have a promising starting value of zT because of its complex structure. The intrinsic phase was successfully synthesized in both bulk powder and single crystal form and indeed has a high zT prior to optimization. A systematic study was done to explore the effects on the electronic structure through isovalent substitution on all three sites where Yb was replaced with Ca, Mn with Cd, and Sb with Bi site. The carrier concentration was also tuned explicitly through aliovalent doping by introducing Na on the Yb site. An increase in the figure of merit was observed when the carrier concentration of the system increased showing large improvements in the case of Na doping and Cd substitution. The structure was analyzed through synchrotron x-ray diffraction and pair distribution function analysis revealing both long range and local disorder. While the aforementioned studies focused on the high temperature physical properties, the structure-property relationship in the low temperature regime was also visited in two compounds: the new Eu5Al3Sb6 phase and the Yb14MSb11 (M = Zn, Mg, Mn) series. The Eu5Al3Sb6 phase represents a new structure type that features Al4 tetrahedra in the solid state. These kinds of triel clusters are rarely seen with only a few examples in the literature making it a great case study for examining the emergent properties of this unique structure. This compound features antiferromagnetic ordering and metallic-like resistivity. Single crystal x-ray diffraction suggests the possibility of a supercell or modulated structure. From electronic band structure calculations, band crossing has been observed indicating that there is potential for this material to exhibit topological behavior. Careful defect tuning can shift the Fermi level to the appropriate energy to induce this behavior. The intermediate valent nature of the Yb14MSb11 (M = Zn, Mg, Mn) compounds have been confirmed through XANES, heat capacity and magnetic susceptibility measurements showing that they are not strictly valence precise compounds as predicted from a Zintl formalism. The three substitutions provide a systematic study on the effect of d electrons within the system where Mg2+ is d0, Mn2+ is d5 and Zn2+is d10. In all cases, there is a small amount of intermediate valency. The amount of intermediate valency is constant for the Mn and Mg compounds while the Zn compound was found to exhibit temperature dependent behavior. This dissertation contains works that are to be published in the future. Some of the work presented here has already been published in part, or full in the following articles: He, A.; Bux, S. K.; Hu, Y.; Uhl, D.; Li, L.; Donadio, D.; Kauzlarich, S. M. Structural Complexity and High Thermoelectric Performance of the Zintl Phase: Yb21Mn4Sb18. Chem. Mater. 2019, 31, 8076 – 8086. He, A.; Cerretti, G.; Kauzlarich, S. M. The impact of site selectivity and disorder on the thermoelectric properties of Yb21Mn4Sb18 solid solutions: Yb21Mn4-xCdxSb18 and Yb21-yCayMn4Sb18. Mater. Adv., 2021, 2, 5764 – 5776. He, A.; Wille, E. L. K; Moreau, L. M.; Thomas, S. M.; Lawrence, J. M.; Bauer, E. D.; Booth, C. H.; Kauzlarich, S. M. Intermediate Yb valence in the Zintl phases Yb14MSb11 (M = Zn, Mn, Mg): XANES, magnetism, and heat capacity. Phys. Rev. Mater., 2020, 4, 114407.

Below are other published manuscripts that I have worked on but are not included in this dissertation:Shang, R.; Nguyen, A. T.; He, A.; Kauzlarich, S. M. Crystal structure characterization and electronic structure of a rare-earth-containing Zintl phase in the Yb-Al-Sb family: Yb3AlSb3. Acta Cryst. C. 2021, C77, 281 – 285.