Engineering Transport Properties in Interconnected Enargite-Stannite Type Cu_2+x Mn_1-x GeS₄ Nanocomposites

Understanding the mechanisms that connect heat and electron transport with crystal structures and defect chemistry is fundamental to develop materials with thermoelectric properties. In this work, we synthesized a series of self-doped compounds Cu_2+x Mn_1-x GeS₄ through Cu for Mn substitution. Using a combination of powder X-ray diffraction, high resolution transmission electron microscopy and precession-assisted electron diffraction tomography, we evidence that the materials are composed of interconnected enargite- and stannite-type structures, via the formation of nanodomains with a high density of coherent interfaces. By combining experiments with ab initio electron and phonon calculations, we discuss the structure-thermoelectric properties relationships and clarify the interesting crystal chemistry in this system. We demonstrate that excess Cu⁺ substituted for Mn²⁺ dopes holes into the top of the valence band, leading to a remarkable enhancement of the power factor and figure of merit ZT.