Conditions for metastable crystallization from undercooled melts

The limits of validity of Ostwald's rule of stages are investigated theoretically in the case of crystallization of undercooled melts. The treatment is within the limits of capillary theory. Two basic models are compared: (1) According to the first one (model A), the phase with lower energy of formation of critical nucleus is predominantly formed. In an enantiotropic-type phase diagram there is no region of homogeneous preferential formation of the low temperature phase. If the phase diagram is monotropic-type there is a certain temperature below which the metastable crystalline phase is preferentially formed. (2) The second assumption takes into consideration that the nature of extremely small phases is somewhat undefined. One certainly cannot determine whether, say 3-particle complex, is of phase 1 or of phase 2. Moreover, it is known that properties of extremely small clusters could be different from the corresponding volume phase. The main assumption is that there is a certain crucial size (n-particle complex) at which the nature of the two phases can be distinguished. Complex of the phase, which has lower chemical potential at the crucial size, will be formed first. According to the model, in the case of enantiotropic-type transition there is a critical temperature. (C) 2004 Elsevier B.V. All rights reserved.