Engineering Transport Properties in Interconnected Enargite‐Stannite Type Cu_2+xMn_1−xGeS₄ Nanocomposites

Abstract

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 1À x GeS 4 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 stannitetype 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 Mn 2 + dopes holes into the top of the valence band, leading to a remarkable enhancement of the power factor and figure of merit ZT.

Copper-Based Diamond-like Thermoelectric Compounds: Looking Back and Stepping Forward

“…Quaternary Cu 2 -II-IV-VI 4 (II = Co, Mn, Hg, Mg, Zn, Cd, Fe; IV = Sn, Ge; VI = Se, S, Te) compounds with more complex tetragonal structures have also been widely studied. The distinguishing features of quaternary CBDL compounds are they possess a wider bandgap and a relatively lower carrier mobility compared with the ternary CBDL compounds [68,70,76,[94][95][96][97][98][99]. Taking the orthorhombic enargite-type Cu 2 MnGeS 4 as an example [95], the bandgap of it is ~1.0 eV in the initial phase while it only converts to 0.9 eV in the Cu 2.5 Mn 0.5 GeS 4 by adjusting the ratio of Mn and Cu atoms.…”

“…The distinguishing features of quaternary CBDL compounds are they possess a wider bandgap and a relatively lower carrier mobility compared with the ternary CBDL compounds [68,70,76,[94][95][96][97][98][99]. Taking the orthorhombic enargite-type Cu 2 MnGeS 4 as an example [95], the bandgap of it is ~1.0 eV in the initial phase while it only converts to 0.9 eV in the Cu 2.5 Mn 0.5 GeS 4 by adjusting the ratio of Mn and Cu atoms. The large-cell Cu 10 B 2 C 4 D 13 [100][101][102][103] (B = Ag, Cu; C = Co, Ni, Zn, Cu, Mn, Fe, Hg, Cd; Q = Sb, Bi, As; Q = Se, S) tetrahedrites have even more complex crystal structures, as shown in Figure 2b,c, respectively.…”