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1. 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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2. Investigation of n-type doping strategies for Mg ₃ Sb ₂

   https://doi.org/10.1039/c8ta03344g  

   Gorai, Prashun; Ortiz, Brenden R.; Toberer, Eric S.; Stevanović, Vladan (January 2018, Journal of Materials Chemistry A)

   Recent, and somewhat surprising, successful n-type doping of Mg 3 Sb 2 was the key to realizing high thermoelectric performance in this material. Herein, we use first-principles defect calculations to investigate different extrinsic n-type doping strategies for Mg 3 Sb 2 and to reveal general chemical trends in terms of dopant solubilities and maximal achievable electron concentrations. In agreement with experiments, we find that Sb substitution is an effective doping strategy, with Se and Te doping predicted to yield up to ∼8 × 10 19 cm −3 electrons. However, we also find that Mg substitution with trivalent (or higher) cations can be even more effective; in particular, the predicted highest achievable electron concentration (∼5 × 10 20 cm −3 ) with La as an extrinsic dopant exceeds that of Se and Te doping. Interstitial doping (Li, Zn, Cu, Be) is found to be largely ineffective either due to self-compensation (Li) or high formation energy (Zn, Cu, Be). Our results offer La as an alternative dopant to Te and Se and reinforce the need for careful phase boundary mapping in achieving high electron concentrations in Mg 3 Sb 2 . 

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3. 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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4. Discovery of multivalley Fermi surface responsible for the high thermoelectric performance in Yb ₁₄ MnSb ₁₁ and Yb ₁₄ MgSb ₁₁

   https://doi.org/10.1126/sciadv.abe9439  

   Perez, Christopher J.; Wood, Maxwell; Ricci, Francesco; Yu, Guodong; Vo, Trinh; Bux, Sabah K.; Hautier, Geoffroy; Rignanese, Gian-Marco; Snyder, G. Jeffrey; Kauzlarich, Susan M. (January 2021, Science Advances)

   null (Ed.)

   The Zintl phases, Yb 14 M Sb 11 ( M = Mn, Mg, Al, Zn), are now some of the highest thermoelectric efficiency p-type materials with stability above 873 K. Yb 14 MnSb 11 gained prominence as the first p-type thermoelectric material to double the efficiency of SiGe alloy, the heritage material in radioisotope thermoelectric generators used to power NASA’s deep space exploration. This study investigates the solid solution of Yb 14 Mg 1− x Al x Sb 11 (0 ≤ x ≤ 1), which enables a full mapping of the metal-to-semiconductor transition. Using a combined theoretical and experimental approach, we show that a second, high valley degeneracy ( N v = 8) band is responsible for the groundbreaking performance of Yb 14 M Sb 11 . This multiband understanding of the properties provides insight into other thermoelectric systems (La 3− x Te 4 , SnTe, Ag 9 AlSe 6 , and Eu 9 CdSb 9 ), and the model predicts that an increase in carrier concentration can lead to zT > 1.5 in Yb 14 M Sb 11 systems. 

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5. Zintl Phase Compounds Mg3Sb2−xBix (x = 0, 1, and 2) Monolayers: Electronic, Phonon and Thermoelectric Properties From ab Initio Calculations

   https://doi.org/10.3389/fmech.2022.876655  

   Chang, Zheng; Ma, Jing; Yuan, Kunpeng; Zheng, Jiongzhi; Wei, Bin; Al-Fahdi, Mohammed; Gao, Yufei; Zhang, Xiaoliang; Shao, Hezhu; Hu, Ming; et al (April 2022, Frontiers in Mechanical Engineering)

   The Mg 3 Sb 2− x Bi x family has emerged as the potential candidates for thermoelectric applications due to their ultra-low lattice thermal conductivity ( κ L ) at room temperature (RT) and structural complexity. Here, using ab initio calculations of the electron-phonon averaged (EPA) approximation coupled with Boltzmann transport equation (BTE), we have studied electronic, phonon and thermoelectric properties of Mg 3 Sb 2− x Bi x (x = 0, 1, and 2) monolayers. In violation of common mass-trend expectations, increasing Bi element content with heavier Zintl phase compounds yields an abnormal change in κ L in two-dimensional Mg 3 Sb 2− x Bi x crystals at RT (∼0.51, 1.86, and 0.25 W/mK for Mg 3 Sb 2 , Mg 3 SbBi, and Mg 3 Bi 2 ). The κ L trend was detailedly analyzed via the phonon heat capacity, group velocity and lifetime parameters. Based on quantitative electronic band structures, the electronic bonding through the crystal orbital Hamilton population (COHP) and electron local function analysis we reveal the underlying mechanism for the semiconductor-semimetallic transition of Mg 3 Sb 2-− x Bi x compounds, and these electronic transport properties (Seebeck coefficient, electrical conductivity, and electronic thermal conductivity) were calculated. We demonstrate that the highest dimensionless figure of merit ZT of Mg 3 Sb 2− x Bi x compounds with increasing Bi content can reach ∼1.6, 0.2, and 0.6 at 700 K, respectively. Our results can indicate that replacing heavier anion element in Zintl phase Mg 3 Sb 2− x Bi x materials go beyond common expectations (a heavier atom always lead to a lower κ L from Slack’s theory), which provide a novel insight for regulating thermoelectric performance without restricting conventional heavy atomic mass approach. 

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