Band convergence in the non-cubic chalcopyrite compounds Cu₂MGeSe₄

Abstract

aInspired by recent theoretical predictions on band convergence in the tetragonal chalcopyrite compounds, we have explored the influence of the crystal structure on the transport and bandstructure of different quaternary chalcopyrites. In theory, a changing lattice parameter ratio of c/2a towards unity should lead to band convergence due to a more cubic and higher symmetry structure. In accordance with this prediction, the different solid solutions explored in this manuscript show a significant impact on the electronic transport depending on the ratio of the lattice parameters. An increasing lattice parameter ratio results in an increase of the carrier effective masses which can be explained by converging bands, ultimately leading to an increase of the power factor and thermoelectric figure of merit in the class of non-cubic chalcopyrite compounds Cu 2 MGeSe 4 . However, the calculations via density functional theory show that the critical value of c/2a, where band convergence occurs, will be different from unity due to symmetry and chemical influences on the band structure.

Electrical and thermal transports of binary copper sulfides CuxS with x from 1.8 to 1.96

“…This further proves that the different sulfur arrangements in Cu x S can induce distinct band structures and density of states near E F . In addition, the relative large m * values observed in Cu x S system are also common features in other Cu/S-or Cu/Se-based systems, e.g., 6.5m e in Cu 2−x Se at 750 K, 16 3.0m e in Cu 3 Sn x Sb 1−x S 4 at 300 K, 31 and 1.2-3.6m e in Cu 2 Zn 1−x Fe x GeSe 4 at 360 K. 32 The large density of states near E F caused by the hybridization of Cu-3d and S/Se-3p orbitals should be responsible for such large m * values in these samples. Fig.…”

On the tuning of electrical and thermal transport in thermoelectrics: an integrated theory–experiment perspective

“…It also suggests a direction of optimisation in these compounds by altering the η towards unity, which can be achieved through doping, composition tuning and solid solutions between compounds with η41 and η o 1, as shown in the experimental efforts in ternary and quaternary chalcogenides. 17,18 In the compounds with multiple bands, the different band evolution with temperature may cause a crossing of band extrema at some point, together with enhanced transport properties. The temperature-induced band convergence has only been studied in PbTe and the related rock-salt IV-VI compounds (Figure 2c).…”

Effective mass and Fermi surface complexity factor from ab initio band structure calculations

“…7 Valley degeneracy (N v ) has been shown to be critical for achieving high zT. [8][9][10][11][12] Many of the best thermoelectric materials are known to have, recently have been found to have, or are being engineered to have high valley degeneracy, including: the lead and tin chalcogenides, 5, 10, 13 diamond like copper selenides, 14,15 Skutterudites, 16 Mg 2 Si, 11,17 Half Heuslers, 18 and Zintl phases. 19 While isolated pockets of Fermi surfaces can improve the quality factor, Fermi surface pockets connected by threads in the lead chalcogenides 20 and the complex Fermi surfaces of bismuth telluride 21 have also been suggested to be beneficial for zT.…”