Far Infrared Phonons of Solid Iodine under Pressure

Far infrared absorption spectra of solid iodine in the region 40-150 cm-1 have been measured up to about 10 GPa, in a diamond anvil cell. The pressure evolution of the frequency position and intensity of the two B1u and B2u lattice modes of the Cmca iodine crystal has been measured for the first time. The much slower increase with pressure of the experimental B1u frequency, comparing with the calculated one, is caused, analogously to the similar behavior of the Ag Raman libron, by the gradual molecular dissociation. The increase with pressure of the intensity of the modes can be correlated to the charge delocalization, preceding the insulator-metal transition.