Incommensurately Modulated Structure in AgCuSe-Based Thermoelectric Materials for Intriguing Electrical, Thermal, and Mechanical Properties

AgCuSe-based materials have attracted great attentions recently in thermoelectric (TE) field due to their extremely high electron mobility, ultralow lattice thermal conductivity, and abnormal brittle-ductile transition at room temperature. However, although the investigation on the crystal structure of AgCuSe low-temperature phase (named as beta-AgCuSe) was started more than half a century before, it is still in controversy yet, which greatly limits the understanding of its intriguing electrical, thermal, and mechanical performance. In this work, via adopting the advanced three-dimensional electron diffraction technique, this study finds that the AgCuSe-based materials crystalize in an incommensurately modulated structure with an orthorhombic Pmmn(0 beta 1/2)s00 superspace group. The local lattice distortion in the incommensurately modulated structure has weak effects on the conduction band minimum due to the delocalized and isotropic feature of Ag 5s states, leading to high carrier mobility. Likewise, the inhomogeneous, weak, and anisotropic Ag-Se bonds result in the high degree of anharmonicity and ultralow lattice thermal conductivity. Furthermore, alloying S in AgCuSe reinforces the interaction between the adjacent Ag-Se layers, yielding the brittle-ductile transition at room temperature. This work well interprets the structure-performance relationship of AgCuSe-based materials and sheds light on the future investigation of this class of promising TE materials.

Automated summary

AgCuSe-based materials have been attracting attention in the field of thermoelectric (TE) recently due to their high electron mobility, low thermal conductivity, and abnormal brittle-ductile transition at room temperature. This study found that the crystal structure of AgCuSe-based materials is incommensurately modulated and has weak local lattice distortion. The delocalized and isotropic feature of Ag 5s states contributes to high carrier mobility, while the inhomogeneous, weak, and anisotropic Ag-Se bonds result in ultralow lattice thermal conductivity. Furthermore, alloying S in AgCuSe strengthens the interaction between adjacent Ag-Se layers, leading to a brittle-ductile transition at room temperature.