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1. Nonadiabatic coupling of the dynamical structure to the superconductivity in YSr ₂ Cu _2.75 Mo _0.25 O _7.54 and Sr ₂ CuO _3.3

   https://doi.org/10.1073/pnas.2018336117  

   Conradson, Steven D.; Geballe, Theodore H.; Jin, Chang-Qing; Cao, Li-Peng; Gauzzi, Andrea; Karppinen, Maarit; Baldinozzi, Gianguido; Li, Wen-Min; Gilioli, Edmondo; Jiang, Jack M.; et al (December 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   A crucial issue in cuprates is the extent and mechanism of the coupling of the lattice to the electrons and the superconductivity. Here we report Cu K edge extended X-ray absorption fine structure measurements elucidating the internal quantum tunneling polaron (iqtp) component of the dynamical structure in two heavily overdoped superconducting cuprate compounds, tetragonal YSr 2 Cu 2.75 Mo 0.25 O 7.54 with superconducting critical temperature, T c = 84 K and hole density p = 0.3 to 0.5 per planar Cu, and the tetragonal phase of Sr 2 CuO 3.3 with T c = 95 K and p = 0.6. In YSr 2 Cu 2.75 Mo 0.25 O 7.54 changes in the Cu-apical O two-site distribution reflect a sequential renormalization of the double-well potential of this site beginning at T c , with the energy difference between the two minima increasing by ∼6 meV between T c and 52 K. Sr 2 CuO 3.3 undergoes a radically larger transformation at T c , >1-Å displacements of the apical O atoms. The principal feature of the dynamical structure underlying these transformations is the strongly anharmonic oscillation of the apical O atoms in a double-well potential that results in the observation of two distinct O sites whose Cu–O distances indicate different bonding modes and valence-charge distributions. The coupling of the superconductivity to the iqtp that originates in this nonadiabatic coupling between the electrons and lattice demonstrates an important role for the dynamical structure whereby pairing occurs even in a system where displacements of the atoms that are part of the transition are sufficiently large to alter the Fermi surface. The synchronization and dynamic coherence of the iqtps resulting from the strong interactions within a crystal would be expected to influence this process. 

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2. The effect of Mg ₃ As ₂ alloying on the thermoelectric properties of n-type Mg ₃ (Sb, Bi) ₂

   https://doi.org/10.1039/D1DT01600H  

   Imasato, Kazuki; Anand, Shashwat; Gurunathan, Ramya; Snyder, G. Jeffrey (July 2021, Dalton Transactions)

   Mg 3 Sb 2 –Mg 3 Bi 2 alloys have been heavily studied as a competitive alternative to the state-of-the-art n-type Bi 2 (Te,Se) 3 thermoelectric alloys. Using Mg 3 As 2 alloying, we examine another dimension of exploration in Mg 3 Sb 2 –Mg 3 Bi 2 alloys and the possibility of further improvement of thermoelectric performance was investigated. While the crystal structure of pure Mg 3 As 2 is different from Mg 3 Sb 2 and Mg 3 Bi 2 , at least 15% arsenic solubility on the anion site (Mg 3 ((Sb 0.5 Bi 0.5 ) 1−x As x ) 2 : x = 0.15) was confirmed. Density functional theory calculations showed the possibility of band convergence by alloying Mg 3 Sb 2 –Mg 3 Bi 2 with Mg 3 As 2 . Because of only a small detrimental effect on the charge carrier mobility compared to cation site substitution, the As 5% alloyed sample showed zT = 0.6–1.0 from 350 K to 600 K. This study shows that there is an even larger composition space to examine for the optimization of material properties by considering arsenic introduction into the Mg 3 Sb 2 –Mg 3 Bi 2 system. 

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3. The role of Co valence in charge transport in the entropy‐stabilized oxide (Mg _0.2 Co _0.2 Ni _0.2 Cu _0.2 Zn _0.2 )O

   https://doi.org/10.1111/jace.18820  

   Jacobson, V.; Huang, J.; Titus, C. J.; Smaha, R. W.; Papac, M.; Lee, S. J.; Zakutayev, A.; Brennecka, G. L. (October 2022, Journal of the American Ceramic Society)

   Abstract Many of the studies on the entropy‐stabilized oxide (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O have been heavily application‐based. Previous works have studied effects of cation stoichiometry on the entropy‐driven reaction to form a single phase, but a fundamental exploration of the effects of anion stoichiometry and/or redox chemistry on electrical properties is lacking. Using near‐edge X‐ray absorption fine structure (NEXAFS) and electrical measurements, we show that oxidizing thin film samples of (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O affects primarily the valence of Co, leaving the other cations in this high‐entropy system unchanged. This oxidation increases electrical conduction in these thin films, which occurs via small polaron hopping mediated by the Co valence shift from 2+ to a mixed 2+/3+ state. In parallel, we show that bulk samples sintered in an oxygen‐rich atmosphere have a lower activation energy for electrical conduction than those equilibrated in a nitrogen (reducing) atmosphere. Combining feasible defect compensation scenarios with electrical impedance measurements and NEXAFS data, we propose a self‐consistent interpretation of Co redox‐mediated small polaron conduction as the dominant method of charge transfer in this system. 

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4. The importance of the Mg–Mg interaction in Mg ₃ Sb ₂ –Mg ₃ Bi ₂ shown through cation site alloying

   https://doi.org/10.1039/C9TA11328B  

   Wood, Max; Imasato, Kazuki; Anand, Shashwat; Yang, Jiong; Snyder, G. Jeffrey (January 2020, Journal of Materials Chemistry A)

   Herein we study the effect alloying Yb onto the octahedral cite of Te doped Mg 3 Sb 1.5 Bi 0.5 has on transport and the material's high temperature stability. We show that the reduction in mobility can be well explained with an alloy scattering argument due to disrupting the Mg octahedral –Mg tetrahedral interaction that is important for placing the conduction band minimum at a location with high valley degeneracy. We note this interaction likely dominates the conducting states across n-type Mg 3 Sb 2 –Mg 3 Bi 2 solid solutions and explains why alloying on the anion site with Bi isn't detrimental to Mg 3 Sb 2 's mobility. In addition to disrupting this Mg–Mg interaction, we find that alloying Yb into the Mg 3 Sb 2 structure reduces its n-type dopability, likely originating from a change in the octahedral site's vacancy formation energy. We conclude showing that while the material's figure of merit is reduced with the addition of Yb alloying, its high temperature stability is greatly improved. This study demonstrates a site-specific alloying effect that will be important in other complex thermoelectric semiconductors such as Zintl phases. 

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5. Experimental and Theoretical Study on the Substitution Patterns in Lithium Germanides: The Case of Li ₁₅ Ge ₄ vs Li ₁₄ ZnGe ₄

   https://doi.org/10.1002/ejic.202100901  

   Osman, Hussien H.; Bobev, Svilen (December 2021, European Journal of Inorganic Chemistry)

   Abstract A new ternary lithium zinc germanide, Li_13.83Zn_1.17(2)Ge₄, was synthesized by a high‐temperature solid state reaction of the respective elements. The crystal structure was determined by single‐crystal X‐ray diffraction methods. The new phase crystallizes in the body‐centered cubic space groupI3d(no. 220) with unit cell parameter of 10.695(1) Å. The crystal structure refinements show that the parent Li₁₅Ge₄structure is stabilized as Li_15−xZn_xGe₄(x≈1) via random substitution of Li atoms by the one‐electron‐richer atoms of the element Zn, by virtue of which the number of valence electrons increases, leading to a more electronically stable system. The substitution effects in the parent Li₁₅Ge₄structure were investigated through both theory and experiment, which confirm that the Zn atoms in this structure prefer to occupy only one of the two available crystallographic sites for Li. The preferred substitution pattern established from experimental results is supported by DFT electronic structure calculations, which also explore the subtleties of the chemical bonding and the electronic properties of the title compounds. 

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