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  1. Abstract

    Yb10MgSb9is a new Zintl compound (with a composition closer to Yb10.5MgSb9) and a promising thermoelectric material first reported in this work. Undoped Yb10MgSb9has an ultralow thermal conductivity due to crystallographic complexity and exhibits a relatively high peak p‐type Seebeck coefficient and high electrical resistivity. This is consistent with Zintl counting and density functional theory (DFT) calculations that the composition Yb10.5MgSb9should be a semiconductor. Na is found experimentally to be an effective p‐type dopant potentially due to the replacement of Na+for Yb2+, allowing for a significant decrease in electrical resistivity. With doping, a dramatic improvement of electrical conductivity is observed and the glass‐like thermal conductivity remains low, allowing for a significant enhancement of the thermoelectric figure of merit,zT. Doping increases thezTfrom 0.23 in undoped Yb10MgSb9to 1.06 in 7 at% Na‐doped Yb10MgSb9at 873K. This high thermoelectric performance found through Na‐doping places this material amongst the leading p‐type Zintl thermoelectrics, making it a promising candidate for future studies and high‐temperature thermoelectric applications.

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  2. While p-type BiCuSeO is a well-known mid-temperature oxide thermoelectric (TE) material, computations predict that superior TE performance can be realized through n-type doping. In this study, we use first-principles defect calculations to show that Cu vacancies are responsible for the native p-type self doping; yet, we find that BiCuSeO is n-type dopable under Cu-rich growth conditions, where the formation of Cu vacancies is suppressed. We computationally survey a broad suite of 23 dopants and find that only Cl and Br are effective n-type dopants. Therefore, we recommend that future experimental doping efforts utilize phase boundary mapping to optimize the electron concentration and resolve the anomalous p–n–p transitions observed in halogen-doped BiCuSeO. The prospect of n-type doping, as revealed by our defect calculations, paves the path for rational design of BiCuSeO chemical analogues with similar doping behavior and even better TE performance. 
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  3. Valley degeneracy is a key feature of the electronic structure that benefits the thermoelectric performance of a material. Despite recent studies which claim that high valley degeneracy can be achieved with inverted bands, our analysis of rock-salt IV–VI compounds using first-principles calculations and k · p perturbation theory demonstrates that mere band inversion is an insufficient condition for high valley degeneracy; rather, there is a critical degree to which the bands must be inverted to induce multiple carrier pockets. The so-called “band inversion parameter” is formalized as a chemically-tunable property, offering a design route to achieving high valley degeneracy in compounds with inverted bands. We predict that the valley degeneracy of rock-salt IV–VI compounds can be increased from N V = 4 to N V = 24, which could result in a corresponding increase in the thermoelectric figure of merit zT . 
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  4. Diamond like semiconductors (DLS) have emerged as candidates for thermoelectric energy conversion. Towards understanding and optimizing performance, we present a comprehensive investigation of the electronic properties of two DLS phases, quaternary Cu 2 HgGeTe 4 and related ordered vacancy compound Hg 2 GeTe 4 , including thermodynamic stability, defect chemistry, and transport properties. To establish the thermodynamic link between the related but distinct phases, the stability region for both is visualized in chemical potential space. In spite of their similar structure and bonding, we show that the two materials exhibit reciprocal behaviors for dopability. Cu 2 HgGeTe 4 is degenerately p-type in all environments despite its wide stability region, due to the presence of low-energy acceptor defects V Cu and Cu Hg and is resistant to extrinsic n-type doping. Meanwhile Hg 2 GeTe 4 has a narrow stability region and intrinsic behavior due to the relatively high formation energy of native defects, but presents an opportunity for bi-polar doping. While these two compounds have similar structure, bonding, and chemical constituents, the reciprocal nature of their dopability emerges from significant differences in band edge positions. A Brouwer band diagram approach is utilized to visualize the role of native defects on carrier concentrations, dopability, and transport properties. This study elucidates the doping asymmetry between two solid-solution forming DLS phases Cu 2 HgGeTe 4 and Hg 2 GeTe 4 by revealing the defect chemistry of each compound, and suggests design strategies for defect engineering of DLS phases. 
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  5. Abstract

    The thermoelectric material ZnSb utilizes elements that are inexpensive, abundant, and viable for mass production. While a high thermoelectric figure of meritzT, is difficult to achieve in Sn‐doped ZnSb, it is shown that this obstacle is primarily due to shortcomings in reaching high enough carrier concentrations. Sn‐doped samples prepared in different equilibrium phase spaces in the ternary Zn‐Sb‐Sn system are investigated using phase boundary mapping, and a range of achievable carrier concentrations is found in the doped samples. The sample with the highestzTin this study, which is obtained with a carrier concentration of 2 × 1019 cm−3when the material is in equilibrium with Zn4Sb3and Sn, confirms that the doping efficiency can be controlled by preparing the doped sample in a particular region of the thermodynamic phase diagram. Moreover, density functional theory calculations suggest that the doping efficiency is limited by the solubility of Sn in ZnSb, as opposed to compensation from native defects. Cognizance of thermodynamic conditions is therefore crucial for rationally tuning the carrier concentration, a quantity that is significant for many areas of semiconductor technologies.

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