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1. The impact of site selectivity and disorder on the thermoelectric properties of Yb ₂₁ Mn ₄ Sb ₁₈ solid solutions: Yb ₂₁ Mn _4−x Cd _x Sb ₁₈ and Yb _21−y Ca _y Mn ₄ Sb ₁₈

   https://doi.org/10.1039/d1ma00497b  

   He, Allan; Cerretti, Giacomo; Kauzlarich, Susan M. (August 2021, Materials Advances)

   Thermoelectric materials can convert heat into electricity. They are used to generate electricity when other power sources are not available or to increase energy efficiency by recycling waste heat. The Yb 21 Mn 4 Sb 18 phase was previously shown to have good thermoelectric performance due to its large Seebeck coefficient (∼290 μV K −1 ) and low thermal conductivity (0.4 W m −1 K −1 ). These characteristics stem respectively from the unique [Mn 4 Sb 10 ] 22− subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb 21 Mn 4− x Cd x Sb 18 ( x = 0, 0.5, 1.0, 1.5) and Yb 21− y Ca y Mn 4 Sb 18 ( y = 3, 6, 9, 10.5) have been prepared, their structures characterized and thermoelectric properties from room temperature to 800 K measured. A detailed look into the structural disorder for the Cd and Ca solid solutions was performed using synchrotron powder X-ray diffraction and pair distribution function methods and shows that these are highly disordered structures. The substitution of Cd gives rise to more metallic behavior whereas Ca substitution results in high resistivity. As both Cd and Ca are isoelectronic substitutions, the changes in properties are attributed to changes in the electronic structure. Both solid solutions show that the thermal conductivities remain extremely low (∼0.4 W m −1 K −1 ) and that the Seebeck coefficients remain high (>200 μV K −1 ). The temperature dependence of the carrier mobility with increased Ca substitution, changing from approximately T −1 to T −0.5 , suggests that another scattering mechanism is being introduced. As the bonding changes from polar covalent with Yb to ionic for Ca, polar optical phonon scattering becomes the dominant mechanism. Experimental studies of the Cd solid solutions result in a max zT of ∼1 at 800 K and, more importantly for application purposes, a ZT avg ∼ 0.6 from 300 K to 800 K. 

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2. Experimental and computational investigations of TiIrB: a new ternary boride with Ti _1+x Rh _2−x+y Ir _3−y B ₃ -type structure

   https://doi.org/10.1515/znb-2021-0121  

   Scheifers, Jan P.; Gibson, Kate A.; Fokwa, Boniface P. (October 2021, Zeitschrift für Naturforschung B)

   Abstract A new ternary phase, TiIrB, was synthesized by arc-melting of the elements and characterized by powder X-ray diffraction. The compound crystallizes in the orthorhombic Ti 1+ x Rh 2− x + y Ir 3− y B 3 structure type, space group Pbam (no. 55) with the lattice parameters a  = 8.655(2), b  = 15.020(2), and c  = 3.2271(4) Å. Density Functional Theory (DFT) calculations were carried out to understand the electronic structure, including a Bader charge analysis. The charge distribution of TiIrB in the Ti 1+ x Rh 2− x + y Ir 3− y B 3 -type phase has been evaluated for the first time, and the results indicate that more electron density is transferred to the boron atoms in the zigzag B 4 units than to isolated boron atoms. 

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3. Green synthesis of reduced Ti ₃ C ₂ T _x MXene nanosheets with enhanced conductivity, oxidation stability, and SERS activity

   https://doi.org/10.1039/C9TC06984D  

   Limbu, Tej B.; Chitara, Basant; Orlando, Jason D.; Garcia Cervantes, Martha Y.; Kumari, Shalini; Li, Qi; Tang, Yongan; Yan, Fei (April 2020, Journal of Materials Chemistry C)

   Transition metal carbides (MXenes) are an emerging family of highly conductive two-dimensional materials with additional functional properties introduced by surface terminations. Further modification of the surface terminations makes MXenes even more appealing for practical applications. Herein, we report a facile and environmentally benign synthesis of reduced Ti 3 C 2 T x MXene (r-Ti 3 C 2 T x ) via a simple treatment with l -ascorbic acid at room temperature. r-Ti 3 C 2 T x shows a six-fold increase in electrical conductivity, from 471 ± 49 for regular Ti 3 C 2 T x to 2819 ± 306 S m −1 for the reduced version. Additionally, we show an enhanced oxidation stability of r-Ti 3 C 2 T x as compared to regular Ti 3 C 2 T x . An examination of the surface-enhanced Raman scattering (SERS) activity reveals that the SERS enhancement factor of r-Ti 3 C 2 T x is an order of magnitude higher than that of regular Ti 3 C 2 T x . The improved SERS activity of r-Ti 3 C 2 T x is attributed to the charge transfer interaction between the MXene surface and probe molecules, re-enforced by an increased electronic density of states (DOS) at the Fermi level of r-Ti 3 C 2 T x . The findings of this study suggest that reduced MXene could be a superior choice over regular MXene, especially for the applications that employ high electronic conductivity, such as electrode materials for batteries and supercapacitors, photodetectors, and SERS-based sensors. 

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4. Correlation-driven electronic reconstruction in FeTe1−xSex

   https://doi.org/10.1038/s42005-022-00805-6  

   Huang, Jianwei; Yu, Rong; Xu, Zhijun; Zhu, Jian-Xin; Oh, Ji Seop; Jiang, Qianni; Wang, Meng; Wu, Han; Chen, Tong; Denlinger, Jonathan D.; et al (December 2022, Communications Physics)

   Abstract Electronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstruction of the Fermi surface tuned independently by two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide family FeTe 1− x Se x . We demonstrate that this reconstruction is driven by the de-hybridization of a strongly renormalized d x y orbital with the remaining itinerant iron 3 d orbitals in the emergence of an orbital-selective Mott phase. Our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase into an orbital-selective Mott phase in d x y as Se concentration is reduced. 

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5. Electronic stabilization by occupational disorder in the ternary bismuthide Li _3–x–y In _x Bi ( x  ≃ 0.14, y  ≃ 0.29)

   https://doi.org/10.1107/S2053229620006439  

   Ovchinnikov, Alexander; Bobev, Svilen (June 2020, Acta Crystallographica Section C Structural Chemistry)

   null (Ed.)

   A ternary derivative of Li 3 Bi with the composition Li 3– x – y In x Bi ( x  ≃ 0.14, y  ≃ 0.29) was produced by a mixed In+Bi flux approach. The crystal structure adopts the space group Fd \overline{3} m (No. 227), with a = 13.337 (4) Å, and can be viewed as a 2 × 2 × 2 superstructure of the parent Li 3 Bi phase, resulting from a partial ordering of Li and In in the tetrahedral voids of the Bi fcc packing. In addition to the Li/In substitutional disorder, partial occupation of some Li sites is observed. The Li deficiency develops to reduce the total electron count in the system, counteracting thereby the electron doping introduced by the In substitution. First-principles calculations confirm the electronic rationale of the observed disorder. 

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