The far-infrared transmission spectrum of p-diiodobenzene was measured as a function of temperature and polarization. Eleven absorption lines were observed at 4 K. Six were assigned, on the basis of their polarization dependences, to vibrational modes of the molecular lattice. The remaining lines were found to be due to either intramolecular modes or Raman-active librational lattice modes which exhibit some infrared activity. Upon heating through the phase transition at 326 K, the two B3u modes disappear and are replaced by another strong band. This is interpreted as due to a structural distortion along the crystalline c axis. No evidence of soft mode behavior near the transition is observed.
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Clayman et al. (1978) studied this question.
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