Long-Range Cationic Order Collapse Triggered by S/Cl Mixed-Anion Occupancy Yields Enhanced Thermoelectric Properties in Cu₅Sn₂S₇

Abstract

To investigate pathways to adjust the charge carrier concentration and optimize the thermoelectric properties, we characterized structural properties, thermal stability, and thermoelectric performance of pristine and Cl-doped Cu5+εSn2−εS7. We demonstrate that Cl doping in Cu5Sn2S7-type monoclinic compounds induces a collapse of the long-range cationic ordering, ultimately leading to a sphalerite-type cubic phase characterized by ordered [Sn­(S,Cl)4] x clusters. The change in crystal structure symmetry upon Cl doping is analyzed by Rietveld refinements against X-ray powder diffraction data, transmission electron microscopy, Mössbauer and X-ray absorption spectroscopy, and low- and high-temperature transport property measurements. The thermoelectric properties of the so-obtained cubic sphalerite Cu5+εSn2−εS7–y Cl y (0 ≤ ε ≤ 0.133, y = 0.35, 0.70) are strongly enhanced with respect to the undoped Cu5Sn2S7: the power factor improves slightly while both electronic and lattice contributions to the thermal conductivity are reduced. Overall, single-phase Cl-doped Cu5.133Sn1.866S7–y Cl y (y = 0.35, 0.70) compounds exhibit high thermoelectric performance, reaching a maximum ZT of 0.45 at 670 K.

Comment on the “Discovery of high-performance thermoelectric copper chalcogenide using modified diffusion-couple high-throughput synthesis and automated histogram analysis technique” by T. Deng, T. Xing, M. K. Brod, Y. Sheng, P. Qiu, I. Veremchuk, Q. Song, T.-R. Wei, J. Yang, G. J. Snyder, Y. Grin, L. Chen and X. Shi, Energy Environ. Sci., 2020, 13, 3041

“…It results in too high L values leading to a substantial underestimation of k L . This difficulty in determining the lattice contribution has been highlighted in the degenerate semiconductor Cu 5 Sn 2 S 7 , 3,7 and several thermoelectric materials, such as SnTe, for which the presence of several electronic bands tends to lower the L values despite the strongly degenerate nature of the compound. [8][9][10][11] To summarize, our present structural investigations clearly demonstrate that the stable structure around the composition Cu 2.1 Sn 0.9 S 3 is not tetragonal with composition Cu 7 Sn 3 S 10 but cubic with the space group P% 43n and the composition Cu 22 Sn 10 S 32 , requiring the revision of the understanding of structure-transport properties relationships of this system.…”

“…Full cationic disorder, induced by Cl-substitution, stoichiometry deviation, or process effect, are well known to strongly enhance phonon scattering and to reduce the lattice thermal conductivity in copper-based sphalerite derivative compounds. 3,4 Attention must be drawn to the extremely low value of the lattice thermal conductivity of the pristine phase. A possible explanation may be in the estimation of the latter by the expression κ L = κ − LσT , where L is the Lorenz number calculated by the single parabolic band (SPB) model.…”

“…It results in too high L values leading to a substantial underestimation of κ L . This difficulty in determining the lattice contribution has been highlighted in the degenerate semiconductor Cu 5 Sn 2 S 7 , 3,7 and several thermoelectric materials, such as SnTe, for which the presence of several electronic bands tends to lower the L values despite the strongly degenerate nature of the compound. 8–11…”

High Thermoelectric Performance in Earth‐Abundant Cu3SbS4 by Promoting Doping Efficiency via Rational Vacancy Design

“…More importantly, the peak zT possesses a higher value among the lead-free TE sulfides without phase transition character (Figure 8a). [9,14,16,[18][19][20]43,44,[47][48][49][50][51][52][53][54] Meanwhile, the zT of this work at 700 K (the maximum zT at the maximum T when the peak zT is lower than 700 K) is also superior to other reported values (Figure S27, Supporting Information).…”

Achieving High Thermoelectric Performance in Mixed Natural‐Synthetic Tetrahedrites

“…Possible strategies to overcome this problem include the design of new materials consisting of earthabundant and environmentally-friendly elements or the direct use of mineral ores without adjustment of their chemical compositions. Fulfilling these requirements, sulfides have drawn attention over the last few years with several families exhibiting high thermoelectric performance with ZT values reaching unity typically around 700 K. Cu 2-x S, [5,6] bornite Cu 5 FeS 4 , [7,8] chalcopyrite CuFeS 2 , [9][10][11] kesterite-like Cu 2 TMS 4 (T = Fe, Co, Ni, Zn; M = Ge, Sn), [12][13][14] colusites Cu 26 T 2 M 6 S 32 (T = Nb, Ta, Cr, Mo, Ti, V, W; M = Ge, Sn, Sb) [15][16][17][18] and Cu 2 SnS 3 [19][20][21][22][23][24][25] are few examples of such systems.…”