Local-Disorder-Induced Low Thermal Conductivity in Degenerate Semiconductor Cu₂₂Sn₁₀S₃₂

Abstract

Sulphur-based semiconductors are attracting attention as environmentally-friendly materials for energy-conversion applications due to their structural complexity and semiconducting properties.Here, we show that the delicate interplay between chemical composition and cationic order/disorder allows to stabilize a new sphalerite derivative phase of cubic symmetry in the Cu-Sn-S diagram: Cu22Sn10S32. Interestingly, its crystal structure is characterized by a semi-ordered cationic distribution, the Cu-Sn disorder being localized on one crystallographic site in a longrange ordered matrix. The origin of the partial disorder as well as its influence on the electronic and thermal transport properties are addressed in detail using a combination of synchrotron diffraction, Mössbauer spectroscopy, transmission electron microscopy, theoretical modelling, and transport properties measurements. These measurements evidence that this compound behaves as a pseudo-gap, degenerate p-type material with very low lattice thermal conductivity (0.5 W m -1 K -1 at 700 K). We show that localized disorder is very effective in lowering 𝜅 ! without compromising the integrity of the conductive framework. Substituting pentavalent Sb for tetravalent Sn is further exploited to lower the hole concentration and doubles the thermoelectric figure of merit ZT to 0.55 at 700 K with respect to the pristine compound. The discovery of this semi-ordered cubic sphalerite derivative Cu22Sn10S32 furthers our understanding of the structureproperties relationships in the Cu-Sn-S system and more generally in ternary and quaternary copper-based systems.

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

“…The latter thus exhibit Cu–S and Sn–S bonds, related to Sn 2 S 7 dimers, longer than that expected for a sphalerite network. , The (Cu + )­S 4 and (Sn 4+ )­S 4 tetrahedra, despite being relatively close in size in a sphalerite-derivative structure, clearly tend to order preferentially because of electrostatic long-range interactions. As a consequence, a large number of different phases can be found in the Cu–Sn–S system, even over a small composition range. − Moreover, small variations in compositions lead to the formation of immiscible secondary Cu–Sn–S phases . Such a feature can make tuning of the transport properties a rather complicated task because of the difficulty of adjusting the carrier concentration in a single-phase solid solution.…”

“…At a fixed Cu/Sn ratio, doping has been attempted through partial substitutions of In, Zn, Ni, Mn, Fe, and Cu for Sn, ,,,− yielding relatively similar outcomes and the formation of at least two different phases. The best performances reported so far have been achieved for the compositions Cu 2 Sn 0.8 Co 0.2 S 3 and Cu 2 Sn 0.85 Fe 0.15 S 3 with maximum ZT values of ca .…”

“…0.75 at 723 K, respectively. Distinct strategies involving nanostructuring have also been successful, reaching a ZT value of 0.64 at 670 K for blended Cu 2 Sn 1‑ x Zn x S 3 nanobulk samples. , Three recent investigations of Cu 2+ x Sn 1– x S 3 have shown that both structural ordering–disordering and thermoelectric properties are highly sensitive to small variations of chemical compositions. − For x ∼ 0.15, an ordered Cu 5 Sn 2 S 7 monoclinic sphalerite-derivative (space group C 2) was synthesized; the latter was shown to be a degenerate semiconductor with a high carrier mobility and a low figure of merit ZT = 0.10 at 700 K. To introduce an additional tuning parameter, Deng et al explored Cl-doped Cu 2.1 Sn 0.9 S 3 reaching a high ZT value, approaching 0.80 at 750 K. Doping with Br in the parent Cu 2.1 Sn 0.9 S 3 has also led to a significant increase in the overall thermoelectric performance, with a ZT value close to unity at 750 K . These interesting preliminary results raise issues regarding the role of Cl for S substitution: does Cl doping modify the crystal structure?…”

Resolution of the Cationic Distribution in Synthetic Germanite Cu22Fe8Ge4S32 by an Experimental Combinatorial Approach Based on Synchrotron Resonant Powder Diffraction Data: A Case Study and Guidelines for Analogous Compounds

“…Although the cationic lattice arrangement effects on the global diffraction pattern are generally quite small, they play a fundamental role in the thermoelectric properties. Indeed, it was shown many times that crystal chemistry and long-range order/disorder phenomena could strongly affect the electronic and thermal properties of the materials. ,− For example, cationic distribution and long-range order/disorder phenomena could strongly affect the electronic ,,,,− and thermal properties ,,,, of sphalerite derivative Cu-based sulfide materials through modification of the chemical bonding and Cu–S conductive network. , …”

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.…”