Optical, Raman and resonance Raman spectra and lattice dynamics calculations of the misfit layer compounds, (SnS) 1.17NbS 2 and (PbS) 1.18TiS 2

Abstract

The UV—visible (340–800 nm), Raman and resonance Raman spectra (10–850 cm −1) of powdered samples of two misfit layer compounds, (SnS) 1.17NbS 2 and (PbS) 1.18TiS 2, at 300 K were investigated. In agreement with X-ray data, vibrational assignments are proposed for most of the internal and external modes of the “TS 2” and “MS” sublattices and the band wavenumbers at Γ ( k=0) are compared to those in the 2H-NbS 2 and 1T-TiS 2 polytypes in order to appreciate the interactions in which “MS” acts as the donor part of the structure. In addition, owing to their incommensurate modulated structures along the a direction, new intense Raman signals are observed and explained by the k=0, ± q selection rules ( q is the modulation vector); such effects are more dominant in the (PbS) 1.18TiS 2 compound in which a stronger structural distortion is effective and a weak but non-negligible charge-transfer mechanism is occurring. New information about the lattice dynamics properties of the “TS 2” sublattices are thus obtained and confirmed by valence force field calculations on the stoichiometric model compounds (SnS) 1.0NbS 2 and (PbS) 1.0TiS 2, which reproduce satisfactorily the essential features of experimental data.