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

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.