Traces of Charge Density Waves in NbS${}_2$

Abstract

Among Transition Metal Dichalcogenides (TMD), NbS${}_2$ is often considered as the archetypal compound that does not have a Charge Density Wave (CDW) in any of its polytypes. By comparison, close iso-electronic compounds such as NbSe${}_2$, TaS${}_2$ and TaSe${}_2$ all have CDW in at least one polytype. Here, we report traces of CDW in the 2H polytype of NbS${}_2$, using diffuse x-ray scattering measurements at 77\,K and room temperature. We observe 12 extremely weak satellite peaks located at $\pm$13${}^{\mathrm{o}}$ 54' from ${\vec{a}}^{\ast }$ and ${\vec{b}}^{\ast }$ around each Bragg peaks in the $(h,k,0)$ plane. These satellite peaks are commensurate with the lattice via $3\vec{q}-\vec{q}{}^{\prime }={\vec{a}}^{\ast }$, where $\vec{q}{}^{\prime }$ is the 120${}^{\circ }$ rotation of $\vec{q}$, and define two chiral $\sqrt{13}a\times \sqrt{13}a$ superlattices in real space. These commensurate wavevectors and tilt angle are identical to those of the CDW observed in the 1T polytype of TaS${}_2$ and TaSe${}_2$. To understand this similarity and the faintness of the peaks, we discuss possible sources of local 1T polytype environment in bulk 2H-NbS${}_2$ crystals.