Dielectric study of phase transitions in 3-nitro-4-chloro-aniline

Abstract

3-nitro-4-chloro-aniline (NCA) exhibiting a phase transition between two solid phases has been studied by frequency domain dielectric spectroscopy. Dielectric studies show that molecular dynamics of NCA changes distinctly at the freezing point (370 K) and at the transition (242 K) from the high-temperature solid phase I(Ph I) to the low-temperature solid phase II(Ph II). In the Ph I just below the freezing point the molecules most probably perform a uniaxial fast reorientation around the pseudohexagonal symmetry axis of the benzene ring. On the other hand upon decreasing temperature in Ph I there are strong and temperature-dependent antiparallel correlations of molecular dipole moments. The correlations are of electrostatic origin and they cause a strong temperature dependence of the Kirkwood correlation factor leading to freezing out intermolecular reorientation. However, in the lower temperature range of the Ph I there is still an enhanced contribution to the static dielectric permittivity originating most probably from anharmonic and anisotropic librations of molecules and/or from intramolecular motion of the NO2 group observed also by x-ray studies. The relaxation process observed in the Ph I-Ph II pretransition region behaves like a soft mode. The relaxation time shows non-Arrhenius behavior in the vicinity of the Ph I-Ph II transition. Below this transition the intramolecular reorientation becomes frozen out leading to antiferroelectric order in the low-temperature Ph II. The phase transition observed is discussed qualitatively in terms of the Fröhlich model of the order-disorder transition. © 1997 American Institute of Physics.