The electronic, vibrational, resonance Raman spectra and the valence force field of the layered NbS 2Cl 2 compound

Abstract

The infrared and Raman spectra (600-10 cm −1) of polycrystalline samples and single crystals of NbS 2Cl 2 were investigated. A complete assignment is proposed and the normal mode wavenumbers in NbS 2Cl 2 and in related niobium compounds are compared. Also, an intense electronic transition at ≈365 nm has been evidenced along with a wide absorption edge in the visible region for this broad-band semiconductor. Therefore, preresonance Raman spectra were investigated using a variety of exciting radiations (λ 0 = 647.1-457.9 nm) and the crude and corrected (from absorption effects of the incident light and of the scattered radiation) excitation profiles of several modes were obtained. In addition, Raman spectra under exact resonance conditions (λ 0 = 363.8 and 351.1 nm) were recorded and analyzed in order to get a better insight into the nature and the symmetry of the resonant electronic transition. Finally, a complete valence force field calculation including intracage (Nb 2S 4), intercage and interlayer contributions has been performed: the potential energy distributions and mean square vibrational amplitudes were thus computed and the force constants in NbS 2Cl 2 and in related compounds were compared. One concludes that the lattice dynamics are governed by strong bonding within the Nb 2S 4 cage units and by weaker but significant interactions in between the cages through Cl bridged atoms to form a two-dimensional system.