Cs-corrected HAADF-STEM observations on the structural modulations caused by charge density wave and Te-vacancy ordering in LaTe<sub>2</sub><sub>−</sub><sub><italic>δ</italic></sub>

Abstract

Microstructural features play a critical role for the understanding of the essential properties of novel functional materials and new devices. Atomic-resolution scanning transmission electron microscopy (STEM) is invaluable for determining the average atomic structure and local structural defects. For strongly correlated electron materials, STEM can be applied to distinguish the structural modulation caused by charge density wave (CDW), chemical ordering (vacancy or impurity atoms). RTe2− δ (R=La, Ce) compounds have attracted recent attention due to their effective low dimensionality. The materials play host to a CDW state above room temperature and can be described in terms of a modulated Cu2Sb-type structure (P4/ nmm ) based on alternating layers of square-planar Te sheets and a corrugated RTe slab. Furthermore, pressure-induced superconductivity in CeTe1.82 with T C of 2.7 K has been reported, suggesting that the nonstoichiometric Te defects are correlated to superconductivity in this material system. Here, we report the study of the structural modulations in LaTe2− δ using STEM. The LaTe2− δ single crystal was grown by self-flux technique. The TEM samples used in the present study were prepared by crushing the well- characterized single crystal, and then the resultant suspensions were dispersed on a holey carbon-covered Cu grid. Electron diffraction experiments were performed in the FEI Tecnai F20 microscope, and HRTEM and high angle annular dark field (HAADF) STEM were performed in the JEOL ARM200F equipped with double aberration correctors and cold field emission gun at room temperature. The experiment result revealing the charge density wave in LaTe2− δ can be tuned by the Te content; the structural modulation correlated with charge density wave can be characterized by a modulation wave vector of q CDW=(1/2− α ) a ∗, where α is the incommensurate parameter determined by the chemical composition. Our experiment data demonstrate that the Cs-corrected HAADF-STEM image can directly reveal the atomic displacements in the Te plane due to electron-phonon coupling. Detailed analysis suggests that the Te atomic displacements adopt an incommensurate wave-pocket structure along each Te-chain with a long periodicity determined by the CDW incommensurability. In addition to the q CDW=(1/2− α ) a ∗ CDW modulation, a superstructure with the vector q 2=1/5(3 a ∗+ b ∗) has been also observed in some regions. In previous study, this modulation was proposed to be correlated with CDW instability, however, recent experimental and theoretical analysis on the electronic structure (FS) shows that there is no such nesting wave vector could be identified. In this paper, our Cs-STEM observations directly demonstrated that the q 2 superstructure actually originates from an imperfect stoichiometry in this layered system. We proposed a 5 × 5 supercell associated with the Te vacancy ordering with the chemical composition of LaTe1.85 based on detailed STEM data analysis. From HRTEM images, it is also noted that the CDW modulation ( q CDW) totally disappears in the region with Te vacancy ordering, suggesting that the CDW can not coexist in the crystals with the Te vacancy ordering. The possible correlation between the ordering of the nonstoichiometric Te defects and the superconductivity in this material system needs to be further investigated.