Traces of charge density waves in NbS 2
Résumé
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 isoelectronic 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 ±13.9 • from a * and b * around each Bragg peak in the (h,k,0) plane. These satellite peaks are commensurate with the lattice via 3 q − − q = = a * , where q is the 120 • rotation of q, and define two √ 13 a × √ 13 a superlattices in real space. These commensurate wave vectors 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.
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