Thermoelectric properties of In1 Co4 Sb12+δ : role of in situ formed InSb precipitates, Sb overstoichiometry, and processing conditions Andrei Novitskiia,b, Alexandra Ivanovab, Illia Serhiienkoa,c, Alexander Burkovd, Takao Moria,c, Vladimir Khovayloa aInternational Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Ibaraki, Tsukuba, 305-0044, Japan bNational University of Science and Technology MISIS (NUST MISIS), Leninsky av. 4, Moscow, 119049, Russia cGraduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Ibaraki, Tsukuba, 305-8573, Japan dIoffe Institute, Politekhnicheskaya st. 26, Saint Petersburg, 194021, Russia NOVITSKII.Andrei@nims.go.jp The 7th Southeast Asia Conference on Thermoelectrics December 7 – 8, 2022

PGEC concept. Skutterudites Phonon glass electron crystal (PGEC) concept: an effective thermoelectric material should conduct electricity effectively as a single crystal conductor and poorly conduct heat like glass [1]. General composition: MT4 Pn12 M is a rare earth, an actinide, an alkaline-earth or an alkaline metal; T is a transition metal of subgroup VIII; Pn is a pnictogen atom. Crystal structure: body-centered cubic (bcc) Space group: Im3 The physics of filled skutterudites, in general, is governed by the interplay of filler ions and the network of the cage. NOVITSKII.Andrei@nims.go.jp 2 fillers pnictogens transition metal Crystal structure of filled skutterudites. Transition metals, pnictogens, and fillers are presented as small, medium-sized, and large spheres, respectively. Visualized with VESTA 3D visualization program [2] [1] Slack, G. in CRC Handbook of Thermoelectrics (ch. 34); CRC Press, 1995 [2] K. Momma, F. Izumi, J. Appl. Crystallogr. 44 (2011) 1272–1276. _

In-filled skutterudites NOVITSKII.Andrei@nims.go.jp 3 Temperature dependence of figure of merit zT for In-filled CoSb3 skutterudites [3-16] Features: - In atoms acting as rattlers in the skutterudite voids of Inx Co4 Sb12 - formation of InSb precipitates when the In solubility limit is exceeded (x ≈ 0.22) - high zT value close to those reported for multidoped skutterudites - formation of the InSb precipitates → CoSb2 impurity phase - different fabrication routes → different microstructure and InSb precipitates of different shape, size, and distribution

Synthesis process Inductive melting of pure elements In, Co and Sb NOVITSKII.Andrei@nims.go.jp 4 annealing at 973K, 5h (vac) ball milling melt spinning ball milling melt spinning spark plasma sintering annealing at different temperatures BMA BM MSA MS BMAG BMG means annealing after IM annealing at 773K, 10h (vac) annealing at 873K, 5h (vac) + extra Sb

Phase composition and microstructure NOVITSKII.Andrei@nims.go.jp 5 XRD patterns for the samples with nominal composition of In1 Co4 Sb12+δ before spark plasma sintering after inductive melting and ball milling (IM + BM), inductive melting, annealing and ball milling (IM + A + BM) XRD patterns of the In1 Co4 Sb12+δ samples prepared by various methods. (b) An enlarged section of (a) in a 2θ range from 21° to 30° where InSb (♦) and Sb (▼) secondary phases have the most intensive reflections SPS XRD patterns for the samples with nominal composition of In1 Co4 Sb12+δ before spark plasma sintering after inductive melting and melt spinning (IM + MS), inductive melting, annealing and melt spinning (IM + A + MS) MS

Phase composition and microstructure NOVITSKII.Andrei@nims.go.jp 6 SEM micrographs of the polished surfaces of the (a) BMG, (b) BMAG, (d) MS, and (e) MSA bulks in backscattered electron channeling contrast mode [17]. TEM images of the (c) BMAG and (f) MSA samples

Electrical transport properties NOVITSKII.Andrei@nims.go.jp 7 Temperature dependence of (a) the electrical conductivity, (b) the Seebeck coefficient, and (c) the power factor for the In1 Co4 Sb12+δ samples prepared by various methods

Thermal conductivity and zT NOVITSKII.Andrei@nims.go.jp 8 Temperature dependence of (a) the total thermal conductivity, (b) the lattice and bipolar thermal conductivities, and (c) the figure of merit for the In1 Co4 Sb12+δ samples prepared by various methods. Solid lines in (b) are the lattice contribution to the thermal conductivity fitted by the power law κlat ∝ T–r 𝑧𝑇 = 𝛼2𝜎 𝜅𝑡𝑜𝑡 𝑇

Conclusions - this work demonstrates that the zT values of Co4 Sb12 based materials can be boosted by proper In doping and Sb overstochiometry - the formation of InSb precipitates and suppression of CoSb2 phases are found to take important parts in modifying the thermoelectric properties NOVITSKII.Andrei@nims.go.jp 9 Temperature dependence of figure of merit zT for the best sample obtained in this work (MSA) and other In-filled CoSb3 skutterudites for comparison [3-16] - utilization of different processing conditions allows one to obtain In1 Co4 Sb12+δ skutterudites with different microstructures and InSb precipitates of different size, shape, and distribution, which, in turn, affects both electrical and thermal transport properties - a maximum zT value of around 1.3 was obtained at 673 K for the sample prepared using inductive melting followed by annealing, melt spinning, and SPS.

References [1] G. Slack, in: CRC Handbook of Thermoelectrics, CRC Press, 1995. [2] K. Momma, F. Izumi, J. Appl. Crystallogr. 44 (2011) 1272–1276. [3] V.V. Khovaylo, T.A. Korolkov, A.I. Voronin, et al., J. Mater. Chem. A 5 (2017) 3541–3546. [4] S. Ghosh, S. Meledath Valiyaveettil, G. Shankar, et al., ACS Appl. Energy Mater. 3 (2020) 635–646. [5] T. He, J. Chen, H.D. Rosenfeld, M.A. Subramanian, Chem. Mater. 18 (2006) 759–762. [6] J. Leszczynski, V. Da Ros, B. Lenoir, et al., J. Phys. D. Appl. Phys. 46 (2013) 495106. [7] R.C. Mallik, C. Stiewe, G. Karpinski, R. Hassdorf, E. Müller, J. Electron. Mater. 38 (2009) 1337–1343. [8] A. Sesselmann, T. Dasgupta, K. Kelm, E. Müller, S. Perlt, S. Zastrow, J. Mater. Res. 26 (2011) 1820–1826. [9] Y. Tang, Y. Qiu, L. Xi, X. Shi, W. Zhang, L. Chen, S.-M. Tseng, S. Chen, G.J. Snyder, Energy Environ. Sci. 7 (2014) 812–819. [10] E. Visnow, C.P. Heinrich, A. Schmitz, et al., Inorg. Chem. 54 (2015) 7818–7827. [11] S. Le Tonquesse, É. Alleno, V. Demange, C. Prestipino, O. Rouleau, M. Pasturel, Mater. Today Chem. 16 (2020) 100223. [12] M. Benyahia, V. Ohorodniichuk, E. Leroy, A. Dauscher, B. Lenoir, E. Alleno, J. Alloys Compd. 735 (2018) 1096–1104. [13] L. Deng, X.P. Jia, T.C. Su, S.Z. Zheng, X. Guo, K. Jie, H.A. Ma, Mater. Lett. 65 (2011) 2927–2929. [14] L. Wang, K.F. Cai, Y.Y. Wang, H. Li, H.F. Wang, Appl. Phys. A 97 (2009) 841–845. [15] N. Gostkowska-Lekner, B. Trawinski, A. Kosonowski, et al., J. Mater. Sci. 55 (2020) 13658–13674. [16] A. Gharleghi, P.-C. Hung, F.-H. Lin, C.-J. Liu, ACS Appl. Mater. Interfaces 8 (2016) 35123–35131. [17] J.I. Goldstein, et al., Scanning Electron Microscopy and X-Ray Microanalysis, Springer New York, 2018. NOVITSKII.Andrei@nims.go.jp 10

Thank you for your attention Andrei Novitskii The study was carried out with financial support from the Russian Science Foundation (project no. 19-79-10282) and JST Mirai JPMJMI19A1. NOVITSKII.Andrei@nims.go.jp slides are available at speakerdeck.com/anovitzkij

Supporting information NOVITSKII.Andrei@nims.go.jp 12 Code Nominal composition Actual composition Phase composition (wt.%) a (Å) d (%) D (nm) ε (%) BMG In1 Co4 Sb12+δ In0.23 Co4 Sb12.39 100% CoSb3 9.044 97 165 ± 50 0.05 ± 0.01 BMAG In0.22 Co4 Sb12.35 95.6% CoSb3 , 3.3% InSb, 1.0% Sb 9.048 98 > 500 < 0.01 BM In0.22 Co4 Sb12.40 100% CoSb3 9.050 93 ~300 ~0.01 BMA In0.23 Co4 Sb12.33 100% CoSb3 + traces of InSb 9.051 95 > 500 ~0.01 MS In0.29 Co4 Sb12.43 99.5% CoSb3 , 0.5% InSb 9.050 95 100 – 300 < 0.01 MSA In0.25 Co4 Sb12.46 100% CoSb3 9.050 93 120 ± 40 0.03 ± 0.01 Code, nominal and actual compositions normalized for 4 Co atoms (from the EDX analysis), phase composition and lattice parameter a (from the Rietveld refinement), density d, crystalline size D, and microdeformation ε of the In1 Co4 Sb12+δ samples prepared by various methods

Supporting information NOVITSKII.Andrei@nims.go.jp 13 Code, Hall carrier concentration, and Hall carrier mobility of the In1 Co4 Sb12+δ samples prepared by various methods Code nH (cm-3) μH (cm2V-1s-1) BMG 1.29∙1020 40.9 BMAG 1.37∙1020 51.4 BM n/a n/a BMA n/a n/a MS 1.89∙1020 36.5 MSA 1.90∙1020 42.1

Supporting information NOVITSKII.Andrei@nims.go.jp 14

Supporting information NOVITSKII.Andrei@nims.go.jp 15 SEM image of the polished surface of the BMG specimen and corresponding EDX maps of the area indicated by the white rectangle. SEM micrograph in electron channeling contrast mode is also shown in the lower right corner

Supporting information NOVITSKII.Andrei@nims.go.jp 16 SEM image of the polished surface of the BMAG specimen and corresponding EDX maps of the area indicated by the white rectangle. SEM micrograph in electron channeling contrast mode is also shown in the lower right corner

Supporting information NOVITSKII.Andrei@nims.go.jp 17 SEM image of the polished surface of the BM specimen and corresponding EDX maps. SEM micrograph in electron channeling contrast mode is also shown in the lower right corner

Supporting information NOVITSKII.Andrei@nims.go.jp 18 SEM image of the polished surface of the BMA specimen and corresponding EDX maps. SEM micrograph in electron channeling contrast mode is also shown in the lower right corner

Supporting information NOVITSKII.Andrei@nims.go.jp 19 SEM image of the polished surface of the MS specimen and corresponding EDX maps of the area indicated by the white rectangle. SEM micrograph in electron channeling contrast mode is also shown in the lower right corner

Supporting information NOVITSKII.Andrei@nims.go.jp 20 SEM image of the polished surface of the MSA specimen and corresponding EDX maps of the area indicated by the white rectangle. SEM micrograph in electron channeling contrast mode is also shown in the lower right corner