Raman spectra of niobium chalcogenide superconductors: NbSe2 and SnNb5Se9

Micro-and FT-Raman spectra of NbSe2 and SnNb5Se9 have been carried out between 5.5, 80 and 300 K. The spectrum of NbSe2 exhibits several features not found previously. A mode at 276 cm(-1) is absent below the CDW transition temperature. The spectrum of SnNb5Se9 presents significant differences allowing the estimation of the force constants K-s, gamma, and beta, 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.Angstrom(-1)) whereas the ratio beta/gamma is considerably reduced from 0.43 to 0.29. Thus the Nb..Se and Se..Se bonds are respectively longer and shorter in SnNb5Se9 than in NbSe2. This suggests that the structure of SnNb5Se9 is more anisotropic than that of NbSe2.