Cs-corrected HAADF-STEM observations on the structural modulations caused by charge density wave and Te-vacancy ordering in LaTe 2 − δ
2017
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). RTe 2 − 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 Cu 2 Sb-type structure
(P4/ nmm ) based on alternating layers of square-planar
Te sheets and a corrugated RTe slab. Furthermore, pressure-induced
superconductivity in CeTe 1.82 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 LaTe 2 − δ using
STEM. The LaTe 2 − 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 LaTe 2 − 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 LaTe 1.85 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.
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