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 fea tures not found previously. A mode at 276 cm(-1) is absent below the C DW transition temperature. The spectrum of SnNb5Se9 presents significa nt differences allowing the estimation of the force constants K-s, gam ma, and beta, corresponding to the interlayer Se-Nb-Se coupling, the S e..Se and the Nb..Se bonds. We find that K-s is unchanged in the new c ompound (0.027 mdyn.Angstrom(-1)) whereas the ratio beta/gamma is cons iderably reduced from 0.43 to 0.29. Thus the Nb..Se and Se..Se bonds a re respectively longer and shorter in SnNb5Se9 than in NbSe2. This sug gests that the structure of SnNb5Se9 is more anisotropic than that of NbSe2.