Disorder in the chalcogen layer in $1T-{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$

The charge density wave (CDW) system $1T\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ was investigated using neutron scattering to determine the nature of the in-plane and out-of-plane disorder. From the local structure analysis, it was deduced that the in-plane star of David motifs are distorted with Se doping just like it was previously observed in $1T\text{−}{\mathrm{TaS}}_2$. This is due to random Ta displacements away from the ideal trigonal symmetry regardless of whether the CDW state is commensurate, nearly commensurate, metallic, or insulating. In the chalcogen layer, the atoms are also distorted with displacements in the perpendicular direction to the planes. The chalcogen distortions are most pronounced in TaSSe that superconducts below 3.6 K in comparison to the distortions observed in ${\mathrm{TaS}}_2$ and ${\mathrm{TaSe}}_2$. The implications of the distortions on superconductivity and the effects on the electron pocket proposed to lead to the superconducting state are not understood at present. Unlike in $1T\text{−}{\mathrm{TaS}}_2$ in which both 13c and 3c layer stacking orders were previously reported, in the nearly commensurate metallic state of $1T$-TaSSe and the commensurate CDW metallic state of ${\mathrm{TaSe}}_2$, no clear evidence for a stacking order is observed down to 2 K.

Figure 1

(a) The unit cell of the high-temperature phase of $1T\text{−}{\mathrm{TaX}}_2$ with the $P\overline{3}\mathrm{m}1$ structure. (b) Below the CDW transition, in-plane and out-of-plane modulations are present. The $\sqrt{13}·\sqrt{13}$ supercell in the CCDW phase gives rise to the star lattice. (c) A schematic for the 13c and 3c stacking orders. (d) The low-temperature DC magnetic susceptibility of $1T$-TaSSe showing the superconducting transition.

Figure 2

The powder neutron diffraction results from NOMAD. The structure function at 2 K for (a) TaSSe and (b) ${\mathrm{TaSe}}_2$ are shown. The data are compared to the average structure model with $P\overline{3}$ symmetry. The temperature dependence of the powder diffraction data in the low-$Q$ region for (c) TaSSe and (d) ${\mathrm{TaSe}}_2$ are shown. The Bragg peaks are labeled as shown.

Figure 3

The temperature dependence of the $G{\left(r\right)}^{\prime }\mathrm{s}$ corresponding to the local structures of (a) TaSSe and (b) ${\mathrm{TaSe}}_2$. No distinctive new features appear as a function of temperature, which indicates the absence of any phase transition in the measured temperature range.

Figure 4

The experimental neutron PDF data at 2 K compared with the calculated PDF based on the average model and the local model for (a) TaSSe and (b) TaSe2. The partial PDFs describing the individual atom pair correlations are shown as well. The in-plane [panels (c) and (d)] and the out-of-plane [panels (e) and (f)] local models for these systems showing the distorted star of David structures within the CDW superlattice at 2 K. The in-plane model shows one superlattice unit cell where distorted stars are centered at the Ta atoms on the corners of the unit cell. The blue hexagons are shown to represent the Ta atom positions in the absence of any distortions.

Figure 5

A comparison of local atomic structure determined from the Fourier transform of the total $S\left(Q\right)$ is shown for $1T\text{−}{\mathrm{TaS}}_2$, TaSSe, and ${\mathrm{TaSe}}_2$. Data are shown at (a) 2 K, (b) 180 K, and (c) 300 K. The $G\left(r\right)$ shifts to higher $r$ as the lattice expands with Se doping, as expected. Overall the local structure shows similar features across composition.

Figure 6

The experimental neutron PDF data (blue) at 2 K is compared with the calculated PDF (green) based on the model with 50% ${\mathrm{TaS}}_2$ and 50% ${\mathrm{TaSe}}_2$. The difference curve is shown in red. The average model is shown in panel (a) and the local model is shown in panel (b).