S C

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IG H

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U C

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1 3 1 I H I G H L I G H T S 2 0 2 2

CuPbBi5S9, which corresponds to gladite mineral [2,3], it was possible to reveal the absence of superstructure diffraction peaks. This indicates that the crystal structure of the synthetic CuPbBi5S9 material can be described by considering disorder between Cu atoms and vacancies. The synthetic CuPbBi5S9 compound does not correspond to an ordered gladite (Figure 123c) but can be described as a copper deficient aikinite Cu1/3□2/3Pb1/3Bi5/3S3 (Figure 123b), with complete disorder between Cu+ and vacancies on the copper site, and between Pb2+and Bi3+ on the Pb site. The disordered structure of the sample was also confirmed by neutron powder diffraction (D2B, ILL) and transmission electron microscopy analyses (Figures 123d-g).

Despite being a nearly intrinsic semiconductor with extremely low carrier concentration (4.5 x 1012 cm-3), this synthetic sulfide is an exceptional prototype for generating high thermoelectric performance, as it can be heavily electron doped by Cl for S substitution, leading to a

thermoelectric figure of merit of 0.43 at 700 K, one of the highest values in Bi-based sulfides.

In addition, the good thermoelectric performance stems from its exceptionally low lattice thermal conductivity. First-principles lattice dynamics calculations (Figure 124) on aikinite indicate the presence of very low energy optical modes associated with Pb and Bi ions and, to a smaller extent, Cu. The Einstein temperatures inferred from the specific heat data, which correspond to wavenumbers of 22.2 and 33.6 cm-1, respectively, are in good agreement with the two lowest peaks observed in the PDOS mainly associated with Pb-weighed thermal motions. This vibrational complexity at low energy hints to substantial anharmonic effects that contribute to lower kL. This is in agreement with the high value of the Grüneisen parameter γ of 1.93, obtained from the sound velocity measurement at 300 K. Such a large value, close to those reported for other ultralow-kL materials, is consistent with strong phonon-phonon scattering in disordered CuPbBi5S9.

PRINCIPAL PUBLICATION AND AUTHORS

A Tunable Structural Family with Ultralow Thermal Conductivity: Copper-Deficient Cu1 x□xPb1 xBi1+xS3, K. Maji (a), P. Lemoine (b), A. Renaud (b), B. Zhang (c,d), X. Zhou (c,d), V. Carnevali (e), C. Candolfi (f), B. Raveau (a), R. Al Rahal Al Orabi (e), M. Fornari (e), P. Vaqueiro (g), M. Pasturel (b), C. Prestipino (b), E. Guilmeau (a), J. Am. Chem. Soc. 144(4), 1846-1860 (2022); https:/doi.org/10.1021/jacs.1c11998 (a) CRISMAT, CNRS, Normandie Univ, ENSICAEN, UNICAEN, Caen (France) (b) Univ. Rennes, CNRS, ISCR-UMR 6226, Rennes (France) (c) College of Physics and Institute of Advanced Interdisciplinary Studies, Chongqing University (China) (d) Analytical and Testing Center of Chongqing University, Chongqing (China) (e) Department of Physics and Science of Advanced Materials Program, Central Michigan University, Michigan (USA) (f) Institut Jean Lamour, UMR 7198 CNRS-Université de Lorraine (France) (g) Department of Chemistry, University of Reading (UK)

REFERENCES

[1] X-L. Shi et al., Chem. Rev. 120, 7399-7515 (2020). [2] V. Syneček et al., Neues Jahrb. Miner. Monatsh. 541-560 (1974). [3] I. Kohatsu et al., Acta Crystallogr. Sect. B. 32, 2401- 2409 (1976).

Fig. 124: Phonon dispersions of aikinite CuPbBiS3, used as proxy for copper-deficient aikinite Cu1/3□2/3Pb1/3Bi5/3S3.