Relationships for single crystal growth during polymorphic transformation

Abstract

It is shown that the polymorphic transformation monocrystal-monocrystal in P-dichlorobenzene proceeds via the growth of a faceted single crystal inside a solid monocrystalline medium consisting of a nonequilibrium phase. There is neither a crystallographic nor any other kind of relationship between the orientation of the growing lattice and the starting phase of p-dichlorobenzene the influence of the initial monocrystalline phase on the growing crystal of the new phase does not differ from the influence of an ‘isotropic’ medium. The transformations are nucleated either at a single site (mono-monocrystal) or at multiple sites. The difference between mono-monocrystal and mono-polycrystal transitions reduces only to the number of nascent centers of crystallization. Despite the arbitrary lattice orientation of the new phase, often there is a ‘memory’ of the previous orientation, which is explained by the conservation of potential nucleation centers at the sites of crystalline defects. A new approach to the measurement of rate of polymorphic transformation is worked out. It is shown that the rate of transformation into a new phase, for a unit volume or a unit area of crystal, is not a physical characteristic of the examined process. Conditions are formulated for an ‘ideal’ experimental basis for measuring transformation rates. The rate of polymorphic transformation V was measured in p-dichlorobenzene as a function of two factors: as a function of the number n of mono-mono transitions and as a function of temperature. A maximum is found in the curve V( n), and the returning of this property after the ‘rest period’ of the crystal. From the curves V( T°) the activation energies were calculated for the processes α → β and β → α, and they are respectively E 1 = 17.4 ± 2.5 and E 2 = 17.1 ± 2.5 kcal/mole.