Superconducting Sn-Intercalated TaSe$_2$: Structural Diversity Obscured by Routine Characterization Techniques

TL;DR

This study reveals pronounced structural heterogeneity in Sn-intercalated TaSe$_2$ that is hidden from routine characterizations. By employing single-crystal X-ray diffraction (SCXRD) and scanning transmission electron microscopy (STEM) on crystals grown from a single Sn:Ta:Se composition ($1:1:2$), the authors identify three global structure types with space groups $R3m$, $P6_3/mmc$, and $Fmm2$, and observe nm-scale local stacking variability within a single crystal. In contrast, powder X-ray diffraction (PXRD), Raman spectroscopy in the standard range, and electronic transport measurements fail to fully resolve or distinguish these structural differences, with only modest sensitivity in low-frequency Raman and no clear correlations between Sn content and $T_ extrm{c}$ or $T_ extrm{CDW}$. The work highlights the need for high-resolution structural tools to accurately characterize intercalated TMDs, as substantial hidden diversity can influence physical properties and our understanding of structure–property relationships. These insights urge reexamination of similar materials to uncover additional phases and phenomena driven by intercalation and stacking in van der Waals layered systems.

Abstract

Using Sn-intercalated TaSe$_2$ as a model system, we demonstrate the presence of structural heterogeneity captured by single-crystal X-ray diffraction (SCXRD) and scanning transmission electron microscopy (STEM) that eludes the routine characterization techniques of powder X-ray diffraction, Raman spectroscopy, and electronic transport measurements. From a single growth composition (1:1:2 Sn:Ta:Se), we obtained crystals diverse in stoichiometry and structure, with near-continuous intercalation for Sn$_x$TaSe$_2$ from $0\lesssim{x}\lesssim1$. Using SCXRD, we found global structural diversity, identifying three new structure types: Sn$_{0.18}$TaSe$_{2.0}$/Sn$_{0.08}$TaSe$_{1.96}$ ($R3m$), Sn$_{0.16}$TaSe$_{2.0}$ ($P6_3/mmc$), and Sn$_{1.2}$TaSe$_{1.9}$ ($Fmm2$). Using STEM, we observed local structural diversity, manifested as regions of highly variable stacking within a single crystal. In contrast, powder X-ray diffraction did not resolve all observed global structures. Raman spectroscopy was unable to distinguish between different structures or compositions in the standard measurement range. Electronic transport measurements showed consistent superconductivity and charge density wave behavior irrespective of Sn-intercalation amount. Our results indicate that routine approaches to characterization of intercalated transition metal dichalcogenides may be inadequate for capturing the diversity of this family of materials, highlighting the need for high-resolution structural characterization when examining the properties of van der Waals-layered compounds.

Figures (6)

• Figure 1: Crystal structures (top), idealized stacking (middle), and rotational disorder (bottom) for Sn$_\textrm{x}$TaSe$_2$. (a) Sn$_{0.18}$TaSe$_{2.0}$ (structure one, dataset one) and Sn$_{0.08}$TaSe$_{1.96}$ (structure one, dataset two). (b) Sn$_{0.16}$TaSe$_{2.0}$ (structure two). (c) Sn$_{1.2}$TaSe$_{1.9}$ (structure three). (d) Ta coordination for Sn$_{0.16}$TaSe$_{2.0}$ as depicted in b. (e) Ta coordination for Sn$_{0.16}$TaSe$_{2.0}$ highlighting the two rotated trigonal prisms arrangements. $\ge$80% occupied sites are designated with a grid pattern. Se$_{180^\circ}$ denotes the Se site attributed to a 180$^\circ$ rotation of the major Ta-centered coordination polyhedron.
• Figure 2: High-resolution cross-sectional STEM for Sn$_x$TaSe$_2$. Periodic regions of (a) Ta-centered trigonal prisms with visible intercalants and (b) Ta-centered trigonal prisms alternated with two channels of Sn. (c) Transition from periodic to variable stacking. (d) Variability in number of TaSe$_2$ layers between Sn channels. STEM images for a, c, and d were obtained from the flake corresponding to structure two. STEM image for b was obtained from the crystal corresponding to structure three. All images show the out of plane direction as vertical.
• Figure 3: Sn$_x$TaSe$_2$ PXRD patterns. (a) Full experimental and theoretical patterns of the crystal structures observed by SCXRD and (b) ($00l$) (or analogous ($h00$)) peaks for the three Sn$_x$TaSe$_2$ structures with equalized peak intensities.
• Figure 4: Raman spectra for Sn$_\textrm{x}$TaSe$_2$. Dashed lines indicate the position of the TaSe$_2$ peaks. Baseline correction and SiO$_2$ subtraction were used for low frequency data; raw spectra are presented in Figures S19-S22.
• Figure 5: Transport behavior of Sn$_\textrm{x}$TaSe$_2$. (a) RRR plotted against the amount of Sn, partitioned according to the presence or absence of a visible CDW and (b) $T_c$ plotted against the Sn amount.