High Power Factors of Thermoelectric Colusites Cu₂₆T₂Ge₆S₃₂ (T = Cr, Mo, W): Toward Functionalization of the Conductive “Cu–S” Network

Abstract

The introduction of hexavalent T6+ cations in p‐type thermoelectric colusites Cu26T2Ge6S32 (T = Cr, Mo, W) leads to the highest power factors among iono‐covalent sulfides, ranging from 1.17 mW m−1 K−2 at 700 K for W to a value of 1.94 mW m−1 K−2 for Cr. In Cu26Cr2Ge6S32, ZT reaches values close to unity at 700 K. The improvement of the transport properties in these new sulfides is explained on the basis of electronic structure and transport calculations keeping in mind that the relaxation time is significantly influenced by the size and the electronegativity of the interstitial T cation. The rationale is based on the concept of a conductive “Cu–S” network, which in colusites corresponds to the more symmetric parent structure sphalerite. A detailed structural analysis of these colusites shows that the distortion of the conductive network is influenced by the presence in the structure of mixed octahedral–tetrahedral [TS4]Cu6 complexes where the T cations are underbonded to sulfur and form metal–metal interactions with copper, Cu–T distances decreasing from 2.76 Å for W to 2.71 Å for Cr. The interactions between these complexes are responsible for the outstanding electronic transport properties. By contrast, the thermal conductivity is not significantly affected.

Enargite Cu3PS4: A Cu–S‐Based Thermoelectric Material with a Wurtzite‐Derivative Structure

“…Similar to the field of photovoltaics, the thermoelectric (TE) community has been stimulated by the emergence of multinary Cu‐based chalcogenides with ZB‐derivative structures as high‐performance materials, such as kesterite/stannite Cu 2 (Zn,Cd)Sn(S,Se) 4 (p‐type), [ 7,8 ] chalcopyrites CuFeS 2 (n‐type), CuGaTe 2 (p‐type), [ 9–11 ] famatinites Cu 3 Sb(S,Se) 4 (p‐type), [ 12,13 ] tetrahedrites Cu 12− x Tr x Sb 4 S 13 (Tr: Mn, Fe, Co, Ni, and Zn) (p‐type), [ 14–16 ] and colusites Cu 26 Tr 2 M 6 S 32 (Tr: V, Nb, Ta, Cr, Mo, or W; M: Ge or Sn) (p‐type). [ 17–20 ] TE materials enable direct conversion from waste heat into electricity, and are therefore expected to play a critical role in realizing a sustainable energy society. Without placing an undue burden on the environment, large‐scale and cost‐effective TE applications require materials to have environmentally benign and low‐cost characteristics alongside high performance, both of which are fulfilled by Cu–S‐based materials.…”

“…[ 28,29 ] As a result, the combination of high S 2 ρ −1 and reduced κ lat leads to a high ZT of 0.5–1.0 at 623–673 K for ZB‐derivative Cu–S‐based materials. [ 15,16,18–20,28–32 ] The Cu–S‐based p‐type materials are expected to be used as counterparts of n‐type materials (e.g., PbTe‐based alloys [ 33,34 ] and half‐Heusler alloys [ 35 ] ) for constructing a Π‐shaped TE power generator.…”

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

“…1.53 mW m À1 K À2 at room temperature and 1.94 mW m À1 K À2 at 700 K. [34] In contrast, the temperature dependence of the electrical resistivity for Mo-and W-solid solutions (0.5 x 1.5) is strikingly different from that of the three limit samples (x = 0 and 2). An upturn of the electrical resistivity leading to as emiconducting behavior (d1/dT < 0) is observed below 375 K for all Mo-and W-solid solutions.W eemphasize that no segregation was observed in the solid solutions within the precision of X-ray diffraction analysis.F rom our theoretical calculations,w ithin the constant relaxation time (t)a pproximation, the changes in band dispersion do not account properly for the temperature dependence of the electrical resistivity,especially for the upturn observed in Figure 5a and Figure 6a ( Figures S7-S10).…”

“…In this context, recent investigations on copper-rich sulfides reveal that these materials form al arge class of p-type thermoelectrics with promising properties.M any copper-based thermoelectric sulfides have indeed been synthesized by several groups:b ornite Cu 5 FeS 4 , [13][14][15] germanite derivative Cu 22 Fe 8 Ge 4 S 32 , [16] stannoidite Cu 8.5 Fe 2.5 Sn 2 S 12 , [17] colusites Cu 26 T 2 M 6 S 32 (T = V, Nb,T a; M = Sn, Ge), [18][19][20][21][22][23] Cu 2 SnS 3 , [24] kesterite Cu 2 ZnSnS 4 , [25,26] and tetrahedrites Cu 12Àx T x Sb 4 S 13 , [27][28][29][30][31][32][33] (T = Mn, Fe,N i, Zn). Among these compounds,the colusites exhibit quite an attractive figure of merit ZT = S 2 T/1k (T being the absolute temperature, S the Seebeck coefficient, 1 the electrical resistivity,a nd k the thermal conductivity), that is, % 0.93 at 675 K. [23] Recently,w es howed that the introduction of hexavalent T 6+ cations in colusites Cu 26 T 2 Ge 6 S 32 (T = Cr, Mo,W )m akes it possible to reach the highest power factors PF (= S 2 /1) reported for iono-covalent sulfides,r anging from 1.15 mW m À1 K À2 at 700 Kf or T = Wt oav alue of 1.94 mW m À1 K À2 for T = Cr without changing significantly the thermal conductivity k. [34] We explained the exceptional transport properties of these sulfides by the presence of interstitial T cations forming mixed tetrahedral-octahedral [TS 4 ]Cu 6 complexes which influence the geometry of the conductive "Cu 26 S 32 "framework inducing,inthis way,various structural distortions.T his model was supported by first principles electronic structure and transport calculations bearing in mind that the size and electronegativity of the T cations may play akey role in those properties.…”

“…In those complexes,a bnormally long iono-covalent T-S bonds compete with short metallic Cu-T interactions and govern the electron transport properties of the conductive "Cu 26 S 32 " framework mainly formed by univalent copper (6d, 12f, 8e sites) and sulfur (8e, 24i sites) atoms. [34] Cu atoms fully occupy the 6d,8e,and 12f sites,Ge atoms occupy the 6c site,w hile the "interstitial" 2a site is shared by either Cr and Mo atoms or Cr and Watoms (Table S1). Thec rystal structure of colusite is completed by the sulfur framework, characterized by the full occupancyoftwo crystallographic sites:8 e and 24i (Table S1).…”