Multipole induced splitting of metal-cage vibrations in crystalline endohedral D_(2d)-M_2C_(84) dimetallofullerenes

摘要

Metal-carbon cage vibrations of crystalline endohedral D_(2d)-M_2C_(84) (M = Sc,Y,Dy) dimetallofullerenes were analyzed by temperature dependent Raman scattering and a dynamical force field model. Three groups of metal-carbon cage modes were found at energies of 35-200 cm~(-1) and assigned to metal-cage stretching and deformation vibrations. They exhibit a textbook example for the splitting of molecular vibrations in a crystal field. Induced dipole-dipole and quadrupole-quadrupole interactions account quantitatively for the observed mode splitting. Based on the metal-cage vibrational structure it is demonstrated that D_(2d)-M_2C_(84) dimetallofullerene retains a monoclinic crystal structure up to 550 K and undergoes a transition from a disordered to an ordered orientational state at a temperature of approximately 150 K.