Key Role of d⁰ and d¹⁰ Cations for the Design of Semiconducting Colusites: Large Thermoelectric ZT in Cu₂₆Ti₂Sb₆S₃₂ Compounds

Abstract

Cu-S-based materials with sphalerite-derivative structures are of interest for their complex cationic distribution, rich crystal structure chemistry, and their potential in energy conversion, and optoelectronic applications. In this study, a new member of colusite, Cu26Ti2Sb6S32, was designed by exploiting the key role of d 0 (T) and d 10 (M) cations in the sphalerite-derivative structure of Cu26T2M6S32 colusites. We succeeded to incorporate d 0 Ti 4+ and d 10 Sb 5+ , into T and M sites respectively, with a tetrahedral coordination rarely found for these two cations in solid-state chemistry. The synthesis produced the first semiconducting compound with the colusite structure. In addition, Cu26Ti2Sb6S32 exhibits a low lattice thermal conductivity. Partial substitution of Ge for Sb increased the hole carrier concentration, leading to enhanced thermoelectric power factor and dimensionless figure of merit (ZT of 0.9 at 673 K). The electronic and phonon structures, responsible for the high thermoelectric performance, were elucidated by first principles calculations.

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

“…order-disorder phenomena) also significantly influence the electronic and thermal conductivity and, consequently,the thermoelectric performance.These parameters are of prime importance,t ogether with doping effects,t o improve/optimise the properties of these materials.T hus,the structure classification is ac omplementary approach to the strategy focused on designing chemical bonds to discover novel functional inorganic copper sulfide materials. A dapted with permission from Bourgsand co-workers, [78] copyright 2018 American Chemical Society, and Hagiwara and co-workers, [44] copyright2 021 American Chemical Society.…”

“…Note that the nature and oxidation state of the interstitial cations in these phases can vary significantly (e.g.with formal charges Cu + ,F e 3+ ,V 5+ ,or W 6+ )i nn atural minerals (Table 2) and can be extended further for synthetic derivatives such as colusites with Ti 4+ , Nb 5+ ,T a 5+ ,C r 6+ ,a nd Mo 6+ cations (Figure 3d). [42][43][44] It is important to mention that the existence of the d 10 cation Cu + in the interstitial position, as reported in the mineral phases omariniite, [35] stannoidite, [36] and germanite, [37] has been questioned, especially in the case of synthetic compounds. [45] The distribution of cations in these structures needs acloser look and are-investigation is currently on-going for the synthetic phase.C onversely,t he metal atoms (M) that compose the environment of the interstitial cation (T)i nt he tetrahedraloctahedral [TS 4 ]M 6 complex are either exclusively Cu or ac ombination of Cu and Fe (Table 2).…”

“…Thetransport properties of complex copper sulfides have been the subject of detailed investigations,i np articular for thermoelectric and photovoltaic applications.Inrecent years, researchers have focused on structure-property relationships, and this strategy is paying off,w ith complex copper sulfides now exhibiting high TE performances.Anon-exhaustive list of some of these copper sulfides and their respective figure of merit (ZT)a re displayed in Figure 7. [27,31,[43][44][45] Tw o families,n amely tetrahedrites and colusites, [10,11] stand out in this figure by their performances approaching af igure of merit of 1, owing to the very low thermal conductivities resulting from their complex crystal structures,w hich are classified in groups Ea nd B, respectively.I ng roup B, the complexity arises from the large unit cells with cationic disorder,w hereas group Ec ompounds exhibit high anisotropic atomic vibrations.These structural features can be used to engineer good thermoelectric materials by adjusting both the electrical and thermal transport properties.T he potential of as olar cell device for photovoltaic applications is illustrated by the light-to-electricity conversion efficiency (theoretically limited at ca. 34 %f or as ingle junction according to the Shockley-Queisser model) and the fill factor (typically ranging from 50 %t o8 0%).…”

“…[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+ ). [7,44] This feature is governed by the particular nature of the T-Cu bond in the complex [TS 4 ]Cu 6 ,w hich is ensured by d-d overlapping of the orbitals of the acceptor d 0 T cations and donor d 10 Cu + cations.…”

“…[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+ ). [7,44] This feature is governed by the particular nature of the T-Cu bond in the complex [TS 4 ]Cu 6 ,w hich is ensured by d-d overlapping of the orbitals of the acceptor d 0 T cations and donor d 10 Cu + cations.…”