Librational modes in solid C60 and doped solid C60.

We present a theoretical study of librational modes in solid C 60 and doped solid C 60 . General results are derived by utilizing the space group symmetries of the solids. At the Brillouin-zone center Γ, in solid C 60 there should be in general five distinct libron frequencies in the simple-cubic phase, and ten in the so-called 2a 0 -fcc phase; in M 3 C 60 , where M=K,Rb, a single libron frequency is expected; in M 3 C 60 , one also expects a single libron frequency