Rate of polymorphic transformation between phases II and III of hexachloroethane

Abstract

The temperature dependent growth rate of the stable phase near the transition point between phases II and III of hexachloroethane (C₂Cl₆) was measured by dilatometry. Reproducible results were obtained by repetitive cycling of the conversions between phases II and III; these conversions being limited to 10% volume changes. The rates of volume change are considered to be proportional to the rectilinear velocity of the phase boundary. The transition temperature was determined to be T_tr= 43.59 ± 0.03°C obtained when the two phases coexisted and the growth rate was zero. The observations show the growth rate to be essentially proportional to the cube of the temperature difference T–T_tr when |T–T_tr| < 1.6°C and it appears to be diverging for |T–T_tr| > 1.6°C.

The results are compared with the prediction of a stochastic theory of the kinetics of phase transitions proposed by Metiu, Kitahara and Ross. While their model equation is applicable only to the description of non-conserved variables, it is the only equation available that admits steady travelling wavefront solutions with non-zero velocity, corresponding to a moving phase boundary. Some restrictions on the model equation are indicated if one attempts to modify it to explain the observed results.