A Lebedeff-type double beam interference microscope has been used to measure the three principal refractive indices of the photoconductor p-diiodobenzene and to observe the behavior of each of these refractive indices as the crystal undergoes a polymorphic phase transition to a structure characterized by markedly lower charge carrier mobility. The refractive index data permit the calculation, for each principal vibration direction, of the change in polarizability accompanying the phase transition. These polarizability results suggest that the phase transition results in a decreased intermolecular orbital overlap which, in turn, narrows the p-type conduction band. This interpretation would account for the sharp drop in hole carrier mobility which accompanies the phase transition.
L. J. Soltzberg, Marion W. Athearn, Judith E. Levy, and Ellen L. Simpson, "Interferometric study of a solid—solid phase transition," Appl. Opt. 16, 3206-3209 (1977)