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Cu2SnS3is the parent compound of a series of phases in the Cu2+xSn1–xS3section (0 ≤ x≤ 0.15) of the ternary Cu–Sn–S phase diagram, the crystal structure of which can be controlled by varying the synthesis process and/or through a fine-tuning of the chemical composition. Despite being structurally close to the sphalerite structure, the thermal transport of these compounds is strongly dependent on the exact lattice symmetry and degree of atomic disorder. Here, we investigate the lattice dynamics of the monoclinic ordered Cu5Sn2S7(space group C2) and cubic disordered Cu5Sn2S6.65Cl0.35(space group F4̅3m) compounds by temperature-dependent powder inelastic neutron scattering (INS). In both cases, the INS spectra feature low-energy optical modes mostly weighed by the thermal motion of Cu atoms. The response of the INS spectra of Cu5Sn2S6.65Cl0.35to temperature variations is indicative of quasi-harmonic behavior. Combined with analyses of the low-temperature specific heat, these findings show that the significant lowering of the lattice thermal conductivity in cubic Cu5Sn2S6.65Cl0.35is due to a reduced phonon mean free path tied to the increased level of disorder in the unit cell. These results highlight how the stabilization of highly symmetric, yet strongly disordered crystal structures akin to those observed in high-entropy alloys, can lead to a drastic reduction in the heat transport, offering an effective approach to design high-performance thermoelectric sulfides.

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]xclusters. 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–yCly(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–yCly(y= 0.35, 0.70) compounds exhibit high thermoelectric performance, reaching a maximum ZT of 0.45 at 670 K.

S-based semiconductors are attracting attention as environmentally friendly materials for energy-conversion applications because of their structural complexity and chemical flexibility. Here, we show that the delicate interplay between the chemical composition and cationic order/disorder allows one 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 semiordered cationic distribution, with the Cu–Sn disorder being localized on one crystallographic site in a long-range-ordered matrix. The origin of the partial disorder and its influence on the electronic and thermal transport properties are addressed in detail using a combination of synchrotron X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy, theoretical modeling, and transport property measurements. These measurements evidence that this compound behaves as a pseudogap, degenerate p-type material with very low lattice thermal conductivity (0.5 W m–1K–1at 700 K). We show that localized disorder is very effective in lowering κLwithout compromising the integrity of the conductive framework. Substituting pentavalent Sb for tetravalent Sn is exploited to lower the hole concentration and doubles the thermoelectric figure of merit ZTto 0.55 at 700 K with respect to the pristine compound. The discovery of this semiordered cubic sphalerite derivative Cu22Sn10S32furthers the understanding of the structure–property relationships in the Cu–Sn–S system and more generally in ternary and quaternary Cu-based systems.

Solid-state synthesis in a sealed silica tube is one of the most popular methods for the production of inorganic materials. Nevertheless, the determination of the best synthesis conditions is a trial-and-error process that generally requires many experiments, varying temperature and duration time. In the present work, the synthesis of a polycrystalline sulfide by solid–liquid–gas reaction in a sealed silica tube is, for the first time, monitored in situ by time-resolved neutron powder diffraction (NPD). All successive crystallization/decomposition sequences and intermediate products were analyzed in detail. These results were combined with ex situ X-ray powder diffraction and thermal analyses on crystallized samples. Our results revealed that sealed tube synthesis is highly dynamic, with reaction times shorter than expected. Importantly, the experiments revealed the primordial role of the cooling conditions on the secondary phase formation and stability. Beyond new data on the complex crystal chemistry of germanite Cu22Fe8Ge4S32, our study demonstrates the compelling benefits of time-resolved in situ NPD in the optimization of the synthesis process for a wide range of complex chalcogenide materials.

A complete study on the thermal stability of the crystal structure and electrical properties, as well as on the phase decomposition, of the high-performance thermoelectric colusite Cu26Cr2Ge6S32is presented. A combination of results from differential scanning calorimetry, thermogravimetric analysis, in situ neutron powder diffraction, and Seebeck coefficient and electrical resistivity temperature cycling experiments show that the high thermoelectric performance colusite Cu26Cr2Ge6S32is stable at least up to 700 K in a nonoxidative atmosphere. A superficial sulfur loss without phase decomposition is observed above ≈760 K, leading to a progressive increase of both electrical resistivity and Seebeck coefficient. The colusite phase, Cu26Cr2Ge6S32, starts to decompose above 830 K into Cu8GeS6and CuCrS2phases. This leads to a Ge enrichment of the colusite phase (i.e., decrease of the Cu/Ge ratio). This initial decomposition step is followed by a second one above 880 K, related to the structural modification from the cubic structure of colusite into the monoclinic Cu2GeS3structure. Above 930 K, the colusite Cu26Cr2Ge6S32phase is fully decomposed. The influence on the crystal structure of the superficial sulfur loss detected a few dozens of degrees before the colusite Cu26Cr2Ge6S32phase decomposition is discussed and compared to the results obtained on the colusite Cu26V2Ge6S32phase, stable at least up to 1000 K. A singular thermal evolution of the unit cell parameter of the latter is observed and discussed. Finally, a discussion on the thermoelectric performance optimization by process-induced structural disordering in these two colusite phases is given.

This study shows that the design of copper-rich sulfides by mimicking natural minerals allows a new germanite-type sulfide Cu22Fe8Ge4S32with promising thermoelectric properties to be synthesized. The Mössbauer spectroscopy and X-ray diffraction analyses provide evidence that the structure of our synthetic compound differs from that of the natural germanite mineral Cu26Fe4Ge4S32by its much higher Cu+/Cu2+ratio and different cationic occupancies. The coupled substitution Cu/Fe in the Cu26–xFe4+xGe4S32series also appears as a promising approach to optimize the thermoelectric properties. The electrical resistivity, which decreases slightly as the temperature increases, shows that these materials exhibit a semiconducting behavior, but are at the border of a metallic state. The magnitudes of the electrical resistivity and Seebeck coefficient increase with x, which suggests that Fe for Cu substitution decreases the hole concentration. The thermal conductivity decreases as the temperature increases leading to a moderately low value of 1.2 W m–1K–1and a maximum ZT value of 0.17 at 575 K.

A univalent copper hyper-stoichiometric stannoidite Cu8+xFe3–xSn2S12with 0 ≤ x≤ 0.5 has been synthesized using mechanical alloying followed by spark plasma sintering. The X-ray diffraction analysis combined with 57Fe and 119Sn Mössbauer investigations has allowed the charge distribution of the cationic species on the various sites to be established and suggests the possibility of a small tin deficiency. The transport properties show a remarkable crossover from a semiconducting to a metal-like behavior as the copper content increases from x= 0 to x= 0.5, whereas correlatively the Seebeck coefficient decreases moderately, with Svalues ranging from 310 to 100 μV/K. The thermal conductivity decreases as the temperature increases showing low values at high temperature, far below those reported in related stannite materials. The investigation of the thermoelectric properties shows that the ZTfigure of merit is dramatically enhanced by the copper hyper-stoichiometry by a factor of 5 going from 0.07 for x= 0 to 0.35 for x= 0.5 at 630 K. This thermoelectric behavior is interpreted on the basis of a model involving the Cu–S framework as the conducting electronic network where the Fe2+/Fe3+species play the role of hole reservoir.