Order-disorder phase-transformation kinetics in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi></mml:math>-chloronitrobenzene studied by nuclear quadrupole resonance

Abstract

$p$-chloronitrobenzene crystallizes in the centrosymmetric space group ${P2}_{1}/c.$ Since the molecule does not possess an inversion point, the crystal exhibit orientational disorder. Recently, it was found that this compound has an ordered phase stable below $282.6\mathrm{K}.$ In the present work, we study the order-disorder phase transformation kinetic using nuclear quadrupole resonance. Isothermal transformation curves for nine different temperatures are analyzed following the theory developed by Cahn. In both the order-disorder and disorder-order phase transformation, nucleation seems to take place at grain edges. The nucleation rate ${}^{v}{I}^{E}$ and growth rate \ensuremath{\gamma} for different temperatures have been determined. We have systematically observed a time dependence for \ensuremath{\gamma} which only remains constant at an early stage of the reaction. This time behavior can be correctly described assuming that the activation energy associated with growth process increases linearly when the nucleus radius reaches a critical value. Experimental data also show that the ordering process is inhibited for temperatures greater than $\ensuremath{\sim}273\mathrm{K}$ (far below the phase-transition point at 282.6 K). This behavior seems to be a consequence of the existence of different microscopic environments in the disordered phase.