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1. Phase Behavior and Superprotonic Conductivity in the System (1– x )CsH 2 PO 4 – x H 3 PO 4 : Discovery of Off-Stoichiometric α-[Cs 1–x H x ]H 2 PO 4

   https://doi.org/10.1021/acs.chemmater.1c04061  

   Wang, Louis S. ; Patel, Sawankumar V. ; Truong, Erica ; Hu, Yan-Yan ; Haile, Sossina M. ( February 2022 , Chemistry of Materials)
2. Intricate Li–Sn Disorder in Rare-Earth Metal–Lithium Stannides. Crystal Chemistry of RE 3 Li 4–x Sn 4+x (RE = La–Nd, Sm; x 7 Li 8–x Sn 10+x ( x ≈ 2.0)

   https://doi.org/10.1021/acs.inorgchem.8b00583  

   Suen, Nian-Tzu ; Guo, Sheng-Ping ; Hoos, James ; Bobev, Svilen ( April 2018 , Inorganic Chemistry)
3. 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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4. Infrared-active phonon modes and static dielectric constants in α -(Al x Ga 1− x ) 2 O 3 (0.18  ≤ x  ≤ 0.54) alloys

   https://doi.org/10.1063/5.0085958  

   Stokey, Megan ; Gramer, Teresa ; Korlacki, Rafał ; Knight, Sean ; Richter, Steffen ; Jinno, Riena ; Cho, Yongjin ; Xing, Huili Grace ; Jena, Debdeep ; Hilfiker, Matthew ; et al ( March 2022 , Applied Physics Letters)

   We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(Al_xGa_1−x)₂O₃alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al₂O₃substrates with x =  0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga₂O₃and α-Al₂O₃. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques.

    

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5. Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α -(Al x Ga 1−x ) 2 O 3 (0≤x≤1)

   https://doi.org/10.1063/5.0087602  

   Hilfiker, Matthew ; Kilic, Ufuk ; Stokey, Megan ; Jinno, Riena ; Cho, Yongjin ; Xing, Huili Grace ; Jena, Debdeep ; Korlacki, Rafał ; Schubert, Mathias ( August 2022 , Applied Physics Letters)

   Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al x Ga 1− x ) 2 O 3 thin films (0 [Formula: see text] x [Formula: see text] 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between α-Ga 2 O 3 and α-Al 2 O 3 , respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga 2 O 3 whereas for α-Al 2 O 3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M 1 type in α-Ga 2 O 3 to M 0 type van Hove singularity in α-Al 2 O 3 . 

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