Raman scattering from layered compounds

Raman spectroscopy has been used to study the layered transition metal dichalcogenides. When in the commensurate-charge-density-wave state, IT-TaS2 and 1T-TaSe2 both show complex Raman spectra. To a first approximation, the observed peaks arise because of the large crystalline unit cell induced by the charge-density wave. Raman spectra taken in the incommensurate phases of 1T-TaS2 and 1T-TaSe2 show only broad and unresolved peaks in the vicinity of 70 cm/1. The Raman spectrum of the 4Hb polytypes, in the region where a charge-density wave exists in the octahedral layers while the trigonal-prismatic layers are normal, is similar to the Raman spectrum in the 1T polytypes. There are some minor modifications apparently due to the changed interlayer coupling. The Raman spectra of 2H-NbSe2 and iron-doped 2H-TaSe2 taken at temperatures where these materials do not exhibit a charge-density-wave state. The observed phonon energies were used in a simple lattice dynamical calculation.