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1. Antibonding induced anharmonicity leading to ultralow lattice thermal conductivity and extraordinary thermoelectric performance in CsK ₂ X (X = Sb, Bi)

   https://doi.org/10.1039/D2TC03356A  

   Yuan, Kunpeng; Zhang, Xiaoliang; Chang, Zheng; Tang, Dawei; Hu, Ming (November 2022, Journal of Materials Chemistry C)

   Full Heusler compounds have long been discovered as exceptional n-type thermoelectric materials. However, no p-type compounds could match the high n-type figure of merit ( ZT ). In this work, based on first-principles transport theory, we predict the unprecedentedly high p-type ZT = 2.2 at 300 K and 5.3 at 800 K in full Heusler CsK 2 Bi and CsK 2 Sb, respectively. By incorporating the higher-order phonon scattering, we find that the high ZT value primarily stems from the ultralow lattice thermal conductivity ( κ L ) of less than 0.2 W mK −1 at room temperature, decreased by 40% compared to the calculation only considering three-phonon scattering. Such ultralow κ L is rooted in the enhanced phonon anharmonicity and scattering channels stemming from the coexistence of antibonding-induced anharmonic rattling of Cs atoms and low-lying optical branches. Moreover, the flat and heavy nature of valence band edges leads to a high Seebeck coefficient and moderate power factor at optimal hole concentration, while the dispersive and light conduction band edges yield much larger electrical conductivity and electronic thermal conductivity ( κ e ), and the predominant role of κ e suppresses the n-type ZT . This study offers a deeper insight into the thermal and electronic transport properties in full Heusler compounds with strong phonon anharmonicity and excellent thermoelectric performance. 

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2. Doping and Anisotropy–Dependent Electronic Transport in Chalcogenide Perovskite CaZrSe ₃ for High Thermoelectric Efficiency

   https://doi.org/10.1002/adts.201900060  

   Osei‐Agyemang, Eric; Enninful_Adu, Challen; Balasubramanian, Ganesh (July 2019, Advanced Theory and Simulations)

   Abstract The potential of an environmentally friendly and emerging chalcogenide perovskite CaZrSe₃for thermoelectric applications is examined. The orthorhombic phase of CaZrSe₃has an optimum band gap (1.35–1.40 eV) for single‐junction photovoltaic applications. The predictions reveal that CaZrSe₃possesses an absorption coefficient of ≈4 × 10⁵cm⁻¹at photon energies of 2.5 eV with an early onset of optical absorption (≈0.2 eV) well below the optimum band gap. Seebeck coefficient,S, is inversely proportional to the carrier mobility as the calculated average effective mass for electrons is higher than for holes;p‐type doping enhances the electrical conductivity, σ. The electronic thermal conductivityκ_eremains low at all temperatures. The upper limit of the thermoelectric figure of merit (ZT_e) attains ≈1.0 when doped at specific chemical potentials, while a high Seebeck coefficient contributes to the ZT_e = 1.95 at 50 K forp‐type doping with 10¹⁸cm⁻³carrier concentration, demonstrating high thermoelectric efficiency. 

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3. Examining the electron transport in chalcogenide perovskite BaZrS ₃

   https://doi.org/10.1039/d1tc00374g  

   Osei-Agyemang, Eric; Koratkar, Nikhil; Balasubramanian, Ganesh (March 2021, Journal of Materials Chemistry C)

   null (Ed.)

   Orthorhombic BaZrS 3 is a potential optoelectronic material with prospective applications in photovoltaic and thermoelectric devices. While efforts exist on understanding the effects of elemental substitution and material stability, fundamental knowledge on the electronic transport properties are sparse. We employ first principles calculations to examine the electronic band structure and optical band gap and interrogate the effect of electron transport on electrical and thermal conductivities, and Seebeck coefficient, as a function of temperature and chemical potential. Our results reveal that BaZrS 3 has a band gap of 1.79 eV in proximity of the optimal 1.35 eV recommended for single junction photovoltaics. An absorption coefficient of 3 × 10 5 cm −1 at photon energies of 3 eV is coupled with an early onset to optical absorption at 0.5 eV, significantly below the optical band gap. The carrier effective mass being lower for electrons than holes, we find the Seebeck coefficient to be higher for holes than electrons. A notable (≈1.0 at 300 K) upper limit to the thermoelectric figure of merit, obtained due to high Seebeck coefficient (3000 μV K −1 ) and ultra-low electron thermal conductivity, builds promise for BaZrS 3 as a thermoelectric. 

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4. Defect Manipulation to Control ZnO Micro-/Nanowire-Metal Contacts

   https://doi.org/DOI: 10.1021/acs.nanolett.8b02892  

   J.W. Cox, G.M. Foster (January 2018, Nano letters)

   Surface states that induce depletion regions are commonly believed to control the transport of charged carriers through semiconductor nanowires. However, direct, localized optical, and electrical measurements of ZnO nanowires show that native point defects inside the nanowire bulk and created at metal−semiconductor interfaces are electrically active and play a dominant role electronically, altering the semiconductor doping, the carrier density along the wire length, and the injection of charge into the wire. We used depth-resolved cathodoluminescence spectroscopy to measure the densities of multiple point defects inside ZnO nanowires, substitutional Cu on Zn sites, zinc vacancy, and oxygen vacancy defects, showing that their densities varied strongly both radially and lengthwise for tapered wires. These defect profiles and their variation with wire diameter produce trap-assisted tunneling and acceptor trapping of free carriers, the balance of which determines the low contact resistivity (2.6 × 10−3 Ω·cm−2) ohmic, Schottky (Φ ≥ 0.35 eV) or blocking nature of Pt contacts to a single nano/microwire. We show how these defects can now be manipulated by ion beam methods and nanowire design, opening new avenues to control nanowire charge injection and transport. 

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5. Symmetry breaking in Ge _1−x Mn _x Te and the impact on thermoelectric transport

   https://doi.org/10.1039/d2ta02347d  

   Adamczyk, Jesse M.; Bipasha, Ferdaushi A.; Rome, Grace Ann; Ciesielski, Kamil; Ertekin, Elif; Toberer, Eric S. (August 2022, Journal of Materials Chemistry A)

   Germanium telluride is a high performing thermoelectric material that additionally serves as a base for alloys such as GeTe–AgSbTe 2 and GeTe–PbTe. Such performance motivates exploration of other GeTe alloys in order understand the impact of site substitution on electron and phonon transport. In this work, we consider the root causes of the high thermoelectric performance material Ge 1− x Mn x Te. Along this alloy line, the crystal structure, electronic band structure, and electron and phonon scattering all depend heavily on the Mn content. Structural analysis of special quasirandom alloy structures indicate the thermodynamic stability of the rock salt phase over the rhombohedral phase with increased Mn incorporation. Effective band structure calculations indicate band convergence, the emergence of new valence band maxima, and strong smearing at the band edge with increased Mn content in both phases. High temperature measurements on bulk polycrystalline samples show a reduction in hole mobility and a dramatic increase in effective mass with respect to increasing Mn content. In contrast, synthesis as a function of tellurium chemical potential does not significantly impact electronic properties. Thermal conductivity shows a minimum near the rhombohedral to cubic phase transition, while the Mn Ge point defect scattering is weak as indicated by the low K L dependence on the Ge–Mn fraction (Fig. 10). From this work, alloys near this phase transition show optimal performance due to low thermal conductivity, moderate effective mass, and low scattering rates compared to Mn-rich compositions. 

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