Raman scattering in PbI2 clusters incorporated into zeolite cages

Raman spectra of PbI2 clusters in zeolite FAU and LTA are reported. A new Raman band with low frequency in the acoustic phonon region was observed in the cluster. The Raman shift was found to increase in inverse proportion to the radius of the cluster. In the optical phonon region, broad and weak Raman signals were observed, in contrast to the sharp and strong Raman bands in the bulk. The experimental results showed that the electron-acoustic phonon coupling via deformation potential interaction was the main process of the electron-phonon interaction due to the localization of the vibrational motion and the wavefunctions of the carriers in a small cluster, while Frohlich coupling to optical phonons became unimportant because of the neutralization of the electronic charge density. The quantized vibrational eigenmodes were calculated for the cluster. The calculated lowest spherical mode was in good agreement with the observed low-frequency Raman shift.