Polymorphic transitions in molecular crystals—III.: Transitions exhibiting unusual behavior

Some polymorphic transitions in molecular crystals, among the few classified earlier as displacive or martensitic on the basis of the existence of a definite orientation relationship between the lattices of the phases and the “instantaneous” transformation rate, have been experimentally investigated. The investigation showed no evidence of a high rate of transition. All occurred through nucleation at the lattice imperfections and continuous growth of the new phase at a rate depending on ΔT = T_tr ? T_a, where T_tr is the actual transition temperature and T_o is the temperature of the thermodynamic equilibrium of the phases. The interface can be observed; its motion can be controlled, be done so slow as it is desirable, or be stopped. The “instantaneous transformations” reported in the literature are caused by temperature delays of nucleation in the absence of appropriate lattice defects. In reality these are polymorphic transitions which appear to be instantaneous only when observed through an optical microscope; actually they occur at the rate corresponding to ordinary temperature dependence. It was established that lamination or cleavage of the crystals in a definite direction is a common feature of the substances exhibiting “atypical” behavior. (Typical behavior has been described in [1,2]). The observed characteristics of the polymorphic transitions (uniform direction of the interfaces, definite orientation relationship of the lattices, small temperature hysteresis of the transition, etc.) can be readily explained in terms of epitaxial growth of the new phase on the cleavage planes of the original phase. At the same time the role of the proximity of crystal structural parameters is elucidated. Simple model calculations showed that a coherent interface may occur when the structural differences in the junction plane of the two phases are small, while in the other cases the contact interface described in [2] is energetically advantageous. However, a cooperative rearrangement at the coherent interface is rejected. There is not any evidence that polymorphic transitions in molecular crystals may occur by means of a “shift”, a “displacement”, a “deformation”, an “overturn”, etc. It is concluded that they always occur as a result of growth. The molecular mechanism of the growth in solids has been described in [2].