Raman spectra of niobium chalcogenide superconductors: NbSe 2 and SnNb 5Se 9

Abstract

Micro- and FT-Raman spectra of NbSe 2 and SnNb 5Se 9 have been carried out between 5.5, 80 and 300 K. The spectrum of NbSe 2 exhibits several features not found previously. A mode at 276 cm −1 is absent below the CDW transition temperature. The spectrum of SnNb 5Se 9 presents significant differences allowing the estimation of the force constants K s, γ, and β, corresponding to the interlayer SeNbSe coupling, the Se..Se and the Nb..Se bonds. We find that K s is unchanged in the new compound (0.027 mdyn.Å −1) whereas the ratio β/γ is considerably reduced from 0.43 to 0.29. Thus the Nb..Se and Se..Se bonds are respectively longer and shorter in SnNb 5Se 9 than in NbSe 2. This suggests that the structure of SnNb 5Se 9 is more anisotropic than that of NbSe 2.