Copper‐Rich Thermoelectric Sulfides: Size‐Mismatch Effect and Chemical Disorder in the [TS₄]Cu₆ Complexes of Cu₂₆T₂Ge₆S₃₂ (T=Cr, Mo, W) Colusites

Abstract

In the present study, we investigate the Mo and W for Cr substitution in the synthetic mineral colusite, Cu26Cr2Ge6S32. Primarily, we elucidate the origin of extremely low electrical resistivity which does not compromise the Seebeck coefficient and leads to outstanding power factors of 1.94 mW m -1 K -2 at 700 K in Cu26Cr2Ge6S32. We demonstrate that the abnormally long iono-covalent T-S bonds competing with short metallic Cu-T interactions govern the electronic transport properties of the conductive "Cu26S32" framework. Additionally, we address the key role of the cationic size-mismatch at the core of the mixed tetrahedraloctahedral complex, illustrated by the cation-size variance, σ 2 , over the transport properties.Using a combination of experimental and theoretical tools, the explanation for the remarkable electrical and thermal transport properties of the Cu26Cr2-xMoxGe6S32 and Cu26Cr2-xWxGe6S32 solid solutions is discussed. In-depth structural analysis using Rietveld refinements of XRPD data reveals two essential effects caused by the substitution of Cr in the solid solutions: (1) Only the tetrahedra that are directly bonded to the [TS4]Cu6 complex are significantly distorted upon substitution and (2) the major contribution to the disorder is localized at the central position of the mixed tetrahedral-octahedral complex, and is maximized for x = 1, i.e. for the highest cationic size-variance, σ 2 . We investigated the low-(5 ≤ T / K ≤ 300) and high-(300 ≤ T / K ≤ 700) temperature electrical and thermal transport properties, including electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall effect (low temperature only) measurements and linked the structural/chemical disorder to the radically different conduction mechanisms in the Cu26Cr2-xMoxGe6S32 and Cu26Cr2-xWxGe6S32 solid solutions.

Insight into the preponderant role of the lattice size in Sn-based colusites for promoting a high power factor

“…59 An interatomic distance T(2 a )–S(8 e ) ≈ 2.228 Å is reported experimentally in the Ge-based colusite, for full Cr occupancy within the T site. 53,58 Fig. 4b shows the simulation of the interatomic distance T(2 a )–S(8 e ) (T(2 a ): V/Cr site) dependence on the lattice parameter.…”

“…The colusite compound family has been extensively investigated within the last 8 years for its promising performance, currently achieving the symbolic unity value of zT , ranking it as one of the best TE sulfide materials. Among the preliminary steps to achieve this good performance, the major roles of the synthesis approaches, 48,50,51 pristine composition, 47,52–54 doping, 50,55–58 and defect chemistry 48,54,59 have been reported and systematically correlated to their influences on the intrinsic crystal, electronic and vibrational structures of the material. This deep understanding of the structure/property relationship in the colusite structure-type enabled the possibility of controlling its native versatile transport properties and building a pathway towards the application stage.…”

Crystal Structure Classification of Copper‐Based Sulfides as a Tool for the Design of Inorganic Functional Materials

“…Thec omplexes affect the transport properties through their remote action on the symmetry of the surrounding tetrahedra that belong to the conductive network. [119] Therefore,t he symmetry and nature of these complexes should be carefully investigated when engineering new materials for energy production. In the high-performance Cr-Ge colusite Cu 26 Cr 2 Ge 6 S 32 an exceptionally high power factor of 1.90-1.94 mW m À1 K À2 at temperatures between 500 Ka nd 700 K was reported.…”

“…[43] Thei nvestigation concluded that the occupation of the "interstitial" site by the smaller,m ore electronegative Cr 6+ cation compared with other Group 6elements, led to less distortion of the conductive Cu-S network while modifying the metallicity through the particular nature of Cr-Cu interactions.T he resulting ZT value of 0.86 at 700 Ki s among the highest reported for acomplex copper sulfide.The impact of the "interstitial" cation on the transport properties was later confirmed through the added notion of cation-size mismatch when the interstitial site was statistically occupied by two dissimilar cations. [119] Theconsequence of such cationic disorder in the "interstitial" position is ac lear change in the conduction mechanism of the materials from at ypical acoustic phonon scattering to an ionised impurity-like scattering mechanism, as demonstrated by low-temperature measurements of the transport properties.Asimilar change is also achieved by providing the structure with extra cations to statistically occupy the interstitial tetrahedral sites not belonging to the superstructure position. [113] In addition, Cu 26 Ti 2 Sb 6 S 32 ,t he first semi-conducting compound with the colusite structure,exhibits an intrinsically low lattice thermal conductivity ranging from 1.6 WK À1 m À1 at 300 Kt o 0.6 WK À1 m À1 at 673 K. [44] High-performance TE properties can be achieved by partial substitution of Sb by Ge,with a ZT value of 0.9 reached at 673 K. [44] From these recent investigations on colusites,i ta ppears that the T site is only occupied by ad 0 cation (Ti 4+ ,V 5+ ,N b 5+ ,T a 5+ ,C r 6+ ,M o 6+ , W 6+ ), whereas the associated M site is occupied exclusively by ac ation with ad 10 configuration similar to Cu + (M = Ge 4+ , Sn 4+ ,S b 5+ ).…”

Ternary MIn2S4 (M = Mn, Fe, Co, Ni) Thiospinels – Crystal Structure and Thermoelectric Properties

“…What makes sulfides perspective candidates for TE applications is the high abundance and low (if any) toxicity of the constituent elements in comparison to Bi 2 Te 3 and PbTe. Besides intensively studied tetrahedrites, chalcosite or colusites, there is an increasing interest in electronic and thermal transport properties of AB 2 S 4 thiospinels . The reason for that is the ability of the spinel‐type structure to accommodate different metal elements,, which results in tunability of physical properties e.g., from insulator to metal .…”