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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 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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4. On the thermal and mechanical properties of Mg _0.2 Co _0.2 Ni _0.2 Cu _0.2 Zn _0.2 O across the high-entropy to entropy-stabilized transition

   https://doi.org/10.1063/5.0122775  

   Rost, Christina M.; Schmuckler, Daniel L.; Bumgardner, Clifton; Bin Hoque, Md Shafkat; Diercks, David R.; Gaskins, John T.; Maria, Jon-Paul; Brennecka, Geoffrey L.; Li, Xiadong; Hopkins, Patrick E. (December 2022, APL Materials)

   As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek a further understanding of the property changes across the phase boundary between “high-entropy” and “entropy-stabilized” phases. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent x-ray diffraction, and nano-indentation. The thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ≈2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24% from 10.8 to 14.1 × 10 −6 K −1 . Mechanical softening is also observed across the transition. 

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5. The bismuth tetramer Bi ₄ : the ν ₃ key to experimental observation

   https://doi.org/10.1039/C8CP03529F  

   Lahm, Mitchell E.; Hoobler, Preston R.; Turney, Justin M.; Peterson, Kirk A.; Schaefer, Henry F. (January 2018, Physical Chemistry Chemical Physics)

   The spectroscopic identification of Bi 4 has been very elusive. Two constitutional Bi 4 isomers of T d and C 2v symmetry are investigated and each is found to be a local energetic minimum. The optimized geometries and vibrational frequencies of these two isomers are obtained at the CCSD(T)/cc-pVQZ-PP level of theory, utilizing the Stoll, Metz, and Dolg 60-electron effective core potential. The fundamental frequencies of the T d isomer are obtained at the same level of theory. The focal point analysis method, from a maximum basis set of cc-pV5Z-PP, and proceeding to a maximum correlation method of CCSDTQ, was employed to determine the dissociation energy of Bi 4 ( T d ) into two Bi 2 and the adiabatic energy difference between the C 2v and T d isomers of Bi 4 . These quantities are predicted to be +65 kcal mol −1 and +39 kcal mol −1 , respectively. Two electron vertical excitation energies between the T d and C 2v electronic configurations are computed to be 156 kcal mol −1 for the T d isomer and 9 kcal mol −1 for the C 2v isomer. The most probable approach to laboratory spectroscopic identification of Bi 4 is via an infrared spectrum. The predicted fundamentals (cm −1 ) with harmonic IR intensities in parentheses (km mol −1 ) are 94(0), 123(0.23), and 167(0) for the T d isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi 4 has yet to be observed. Through natural bond orbital analysis, the C 2v isomer of Bi 4 was discovered to exhibit “long-bonding” between the furthest apart ‘wing’ atoms. This long-bonding is postulated to be facilitated by the σ-bonding orbital between the ‘spine’ atoms of the C 2v isomer. 

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