On the phase transition in N-isopropylcarbazole

Abstract

The elastic properties of N-isopropylcarbazole (NIPC), a pyroelectric molecular crystal, are investigated by Brillouin scattering. The full elastic constant tensor is determined at 295 K and the temperature dependences of the elastic constants are given. The major experimental finding reported is the anomaly of the LA a-axis mode governed by the c11 elastic constant. This mode exhibits a pronounced downward bending on both sides of the nonferroic, first order phase transition at ∼137 K. Symmetry allowed linear-quadratic and biquadratic couplings between the Brillouin zone boundary one-dimensional order parameter and the zone center strain introduced into the Landau free energy do not account for the observed anomaly. The transition is characterized in terms of a strong dispersion of the c11 elastic constant and a large dynamical critical behavior. By combining the c11 elastic constant data and the Brillouin scattering LA a-axis mode half-width data through a Landau–Khalatnikov process, one can extract a relaxation time satisfying a mean-field dependence characteristic of critical slowing down of the order parameter.