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1. Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

   https://doi.org/10.1002/anie.202301176  

   Chanakian, Sevan ; Peng, Wanyue ; Meschke, Vanessa ; Ashiquzzaman_Shawon, A_K_M ; Adamczyk, Jesse ; Petkov, Valeri ; Toberer, Eric ; Zevalkink, Alexandra ( June 2023 , Angewandte Chemie International Edition)

   AMXcompounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on theM‐site in the planarMX‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu_1−xZn_0.5xSb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn_0.5Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on theMand Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK⁻¹at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximumzTincreases from 0.09 to 0.7 as we go from EuCuSb to EuZn_0.5Sb.

    

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2. Glass‐Like Through‐Plane Thermal Conductivity Induced by Oxygen Vacancies in Nanoscale Epitaxial La 0.5 Sr 0.5 CoO 3− δ

   https://doi.org/10.1002/adfm.201704233  

   Wu, Xuewang ; Walter, Jeff ; Feng, Tianli ; Zhu, Jie ; Zheng, Hong ; Mitchell, John F. ; Biškup, Neven ; Varela, Maria ; Ruan, Xiulin ; Leighton, Chris ; et al ( November 2017 , Advanced Functional Materials)

   Ultrafast time‐domain thermoreflectance (TDTR) is utilized to extract the through‐plane thermal conductivity (Λ_LSCO) of epitaxial La_0.5Sr_0.5CoO_3−δ(LSCO) of varying thickness (<20 nm) on LaAlO₃and SrTiO₃substrates. These LSCO films possess ordered oxygen vacancies as the primary means of lattice mismatch accommodation with the substrate, which induces compressive/tensile strain and thus controls the orientation of the oxygen vacancy ordering (OVO). TDTR results demonstrate that the room‐temperatureΛ_LSCOof LSCO on both substrates (1.7 W m⁻¹K⁻¹) are nearly a factor of four lower than that of bulk single‐crystal LSCO (6.2 W m⁻¹K⁻¹). Remarkably, this approaches the lower limit of amorphous oxides (e.g., 1.3 W m⁻¹K⁻¹for glass), with no dependence on the OVO orientation. Through theoretical simulations, origins of the glass‐like thermal conductivity of LSCO are revealed as a combined effect resulting from oxygen vacancies (the dominant factor), Sr substitution, size effects, and the weak electron/phonon coupling within the LSCO film. The absence of OVO dependence in the measuredΛ_LSCOis rationalized by two main effects: (1) the nearly isotropic phononic thermal conductivity resulting from the imperfect OVO planes when δ is small; (2) the missing electronic contribution toΛ_LSCOalong the through‐plane direction for these ultrathin LSCO films on insulating substrates.

    

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3. Exceptionally high electronic mobility in defect-rich Eu 2 ZnSb 2−x Bi x alloys

   https://doi.org/10.1039/C9TA14170G  

   Chanakian, Sevan ; Uhl, David ; Neff, David ; Drymiotis, Fivos ; Park, Junsoo ; Petkov, Valeri ; Zevalkink, Alexandra ; Bux, Sabah ( March 2020 , Journal of Materials Chemistry A)

   The Zintl compound Eu 2 ZnSb 2 was recently shown to have a promising thermoelectric figure of merit, zT ∼ 1 at 823 K, due to its low lattice thermal conductivity and high electronic mobility. In the current study, we show that further increases to the electronic mobility and simultaneous reductions to the lattice thermal conductivity can be achieved by isovalent alloying with Bi on the Sb site in the Eu 2 ZnSb 2−x Bi x series ( x = 0, 0.25, 1, 2). Upon alloying with Bi, the effective mass decreases and the mobility linearly increases, showing no signs of reduction due to alloy scattering. Analysis of the pair distribution functions obtained from synchrotron X-ray diffraction revealed significant local structural distortions caused by the half-occupied Zn site in this structure type. It is all the more surprising, therefore, to find that Eu 2 ZnBi 2 possesses high electronic mobility (∼100 cm 2 V −1 s −1 ) comparable to that of AM 2 X 2 Zintl compounds. The enormous degree of disorder in this series gives rise to exceptionally low lattice thermal conductivity, which is further reduced by Bi substitution due to the decreased speed of sound. Increasing the Bi content was also found to decrease the band gap while increasing the carrier concentration by two orders of magnitude. Applying a single parabolic band model suggests that Bi-rich compositions of Eu 2 ZnSb 2−x Bi x have the potential for significantly improved zT ; however, further optimization is necessary through reduction of the carrier concentration to realize high zT . 

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4. Effect of electron- and hole-doping on properties of kagomé-lattice ferromagnet Fe 3 Sn 2

   https://doi.org/10.1088/1361-648X/acc91e  

   Adams, Milo ; Huang, Chen ; Shatruk, Michael ( April 2023 , Journal of Physics: Condensed Matter)

   We report a theoretical investigation of effects of Mn and Co substitution in the transition metal sites of the kagomé-lattice ferromagnet, Fe₃Sn₂. Herein, hole- and electron-doping effects of Fe₃Sn₂have been studied by density-functional theory calculations on the parent phase and on the substituted structural models of Fe_3−xM_xSn₂(M = Mn, Co;x= 0.5, 1.0). All optimized structures favor the ferromagnetic ground state. Analysis of the electronic density of states (DOS) and band structure plots reveals that the hole (electron) doping leads to a progressive decrease (increase) in the magnetic moment per Fe atom and per unit cell overall. The high DOS is retained nearby the Fermi level in the case of both Mn and Co substitutions. The electron doping with Co results in the loss of nodal band degeneracies, while in the case of hole doping with Mn emergent nodal band degeneracies and flatbands initially are suppressed in Fe_2.5Mn_0.5Sn₂but re-emerge in Fe₂MnSn₂. These results provide key insights into potential modifications of intriguing coupling between electronic and spin degrees of freedom observed in Fe₃Sn₂.

    

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5. High Thermoelectric Performance in PbSe–NaSbSe 2 Alloys from Valence Band Convergence and Low Thermal Conductivity

   https://doi.org/10.1002/aenm.201901377  

   Slade, Tyler J. ; Bailey, Trevor P. ; Grovogui, Jann A. ; Hua, Xia ; Zhang, Xiaomi ; Kuo, Jimmy Jiahong ; Hadar, Ido ; Snyder, G. Jeffrey ; Wolverton, Chris ; Dravid, Vinayak P. ; et al ( July 2019 , Advanced Energy Materials)

   PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe₂(NaPb_mSbSe_m+2) are described and the most promising p‐type PbSe‐based thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPb_mSbSe_m+2exhibits unorthodox semiconducting‐like electrical conductivity, despite possessing degenerate carrier densities of ≈10²⁰cm⁻³. It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe₂in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe₂lowers the energy separation between the L‐ and Σ‐valence bands and enhances the power factors under 700 K. The crystallographic disorder of Na⁺, Pb²⁺, and Sb³⁺moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1–0.55 W m^‐1K^‐1between 400 and 873 K without nanostructures. As a consequence, NaPb₁₀SbSe₁₂achieves maximumZT≈1.4 near 900 K when optimally doped. More importantly, NaPb₁₀SbSe₁₂maintains highZTacross a broad temperature range, giving an estimated recordZT_avgof ≈0.64 between 400 and 873 K, a significant improvement over existing p‐type PbSe thermoelectrics.

    

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