Raman study of orientational disorder in the molecular charge-transfer crystals naphthalene–TCNB and naphthalene–PMDA

Abstract

We have studied orientational disorder in the molecular charge-transfer crystals naphthalene–tetracyanobenzene (N–TCNB) and naphthalene–pyromellitic dianhydride (N–PMDA) by Raman scattering from low frequency intermolecular modes. From the temperature dependence of intensities and linewidths we find strong evidence for an order–disorder transition in naphthalene–TCNB at 63 K in which the center of symmetry is removed. In naphthalene–PMDA, on the other hand, the spectrum is consistent with an ordered structure in which the molecules exhibit anharmonic motion at high temperatures.