Staging and interlayer interaction in the misfit-layer compounds (RS)nNbS2(R=La,Ce;n=0.6,1.2) studied by Raman and infrared spectroscopies

Raman-scattering and infrared reflection measurements on misfit-layer compounds (RS${)}_{\mathrm{n}}$ ${\mathrm{NbS}}_2$ (n=0.6,1.2; R=La,Ce) have been carried out in order to investigate the interlayer interaction and its relation to the charge transfer from the RS layer to the ${\mathrm{NbS}}_2$ layer. It is found that the intralayer modes of the ${\mathrm{NbS}}_2$ layer in (RS${)}_{\mathrm{n}}$ ${\mathrm{NbS}}_2$ are considerably upshifted from those of a pristine ${\mathrm{NbS}}_2$ crystal. Further, some extra modes are observed. These facts indicate a strong interlayer interaction in these compounds. The modes corresponding to the intralayer vibrations of the ${\mathrm{NbS}}_2$ layer are less perturbed for the stage-2 (n=0.6) compounds than for the stage-1 (n=1.2) compounds owing to the difference of the amount of the charge transfer. However, the ${\mathrm{A}}_2^{\prime }$ (421 ${\mathrm{cm}}^{\mathrm{-}1}$) mode, which is Raman inactive in an isolated ${\mathrm{NbS}}_2$ layer, appears strongly for the stage-2 compounds. This result is interpreted in terms of the breakdown of the mirror symmetry of the ${\mathrm{NbS}}_2$ layer in the stage-2 compounds caused by the intercalation of the RS layer.