Fe3+-Induced Disorder and Magnetism in Thermoelectric Copper Sulfides

Copper-rich sulfides have emerged in the past decade as serious candidates for cost-efficient and environmentally friendly thermoelectric applications. In the present work, we succeeded in introducing Fe3+ (d5 electronic configuration) into interstitial sites within the sphalerite framework, typically occupied by d0 cations, to create germanite-inspired colusite-type structures. The Cu26-xFe2+xGe6S32 (0 ≤ x ≤ 4) series retains the parent cubic symmetry characteristic of both colusite and germanite, while progressively approaching the Fe-rich cation distribution of synthetic germanite Cu22Fe8Ge4S32. High-resolution X-ray diffraction and transmission electron microscopy show that the cubic sphalerite-derived framework (space group P4̅3n) is preserved, while increasing Fe content induces controlled disorder on the mixed Cu/Fe 12f site. 57Fe Mössbauer spectroscopy confirms the oxidation state of Fe3+ cations and their preferred occupation at both the interstitial 2a site and the surrounding 12f position, generating a wide distribution of local environments arising from the mixed occupancy in [FeS4](Cu,Fe)6 tetrahedral–octahedral complexes. First-principles calculations based on density functional theory confirm the preferred substitution of Cu by Fe on the 12f site, as well as a favored Fe clustering in Cu26-xFe2+xGe6S32 for high x values. This engineered disorder markedly reduces the lattice thermal conductivity from 3.2 W m–1 K–1 (x = 0) to 1.4 W m–1 K–1 (x = 4) at room temperature, while the concomitant tuning of the Cu2+/Cu+ ratio optimizes carrier concentration and Seebeck coefficient, leading to a peak zT of 0.42 at 673 K for x = 3. Magnetic measurements reveal soft ferromagnetic-like behavior and unusually low effective moments, pointing to strong Fe–S hybridization and intersite interactions. This work demonstrates that Fe3+ (d5 cation) can occupy interstitial sites and induce mixed occupancy within the sphalerite-derived network, revealing a strategy for coupling controlled structural disorder, phonon scattering, and carrier transport in copper-rich sulfide thermoelectrics.