Abstract:

The structure of low-temperature α-Cu₂Se, which is of great importance for understanding the mechanism of the significant increase in thermoelectric performance during the α-β phase transition of Cu₂Se, has still not been fully solved. Because it is restricted by the quality of polycrystal and powder specimens and the accuracy of characterization methods such as the conventional transmission electron microscopy (TEM) and X-ray diffraction (XRD), direct observation with atomic-scale resolution to reveal the structural details has not been realized, although electron diffraction and high-resolution transmission electron microscopy (HRTEM) studies have indicated the complexity of the α-Cu₂Se layered structure. Owing to developments in the focused ion beam (FIB) milling preparation method, high-quality single crystalline specimens with specific crystallographic orientations can be prepared to ensure that atomic-resolution images along a specific orientation can be acquired. Furthermore, the developments in aberration correction technology in TEM and scanning transmission electron microscopy (STEM) allow us to observe the subtle details of structural variation and evolution. Herein, we report, for the first time, the atomic-resolution high-angle annular dark field (HAADF) images acquired along the $ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$ axis of α-Cu₂Se using spherical-aberration (C_s)-corrected STEM from FIB-prepared single crystalline specimens. The observations revealed that the complex structure is generated by ordered fluctuations of Se atoms with various forms, including that some of the Se atoms on the two sides of the Cu deficiency layer get closer to each other than the others and the neighboring Cu deficiency layers have different forms of ordered Se fluctuations. These characteristics can only be observed along the $ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$ axis, while these details were not visible in a previous study along the $ <\bar{1}10{{>}_{\text{c}}} $ axis or in our results obtained along the ${{\left[ \bar{1}01 \right]}_{\text{c}}} $ axis. By combining the electron diffraction patterns, several models of the unit cell variants were established, including the two-layer and four-layer cells (both have two different shapes) and the two-layer variants with and without central symmetry. These variants can also transform into each other, and an α-Cu₂Se crystal can be formed through the random assembly of these variants. Using the program QSTEM, the corresponding HAADF images of these variants were simulated. The simulation results were similar to the experimental HAADF images and reflected most of the observed details, including the different forms of the ordered fluctuations of Se atoms and the dispersion of Cu atoms, which indicates that our structure models of α-Cu₂Se are reasonable. This work provides new critical information for thoroughly understanding the structure of α-Cu₂Se and the α-β phase transition of Cu₂Se.

Key words: α-Cu₂Se, Focused ion beam, Electron diffraction, C_s-correction, STEM, Ordered fluctuation