Abstract
Undercooling a melt often facilitates a metestable phase to preferentially nucleate. In the present paper, the formation of a metastable phase from undercooled melts was developed from the point of the competitive nucleation criterion. The classical nucleation theory shows that the most critical factor for forming a critical nucleus is the interface free energy g. In fact, on the simple liquid such as the melt of a mono-atomic metal, Spaepen's negentropic model suggests that a scaling g with the entropy of fusion is to be the decisive factor for forming the critical nucleus. However, recent numerical simulations such as the molecular dynamics or density functional theory show ambiguous relations between a and the crystal structures. Furthermore, in compound materials such as oxides, in which polyhedrons of oxygen are the structural units both in the solid and liquid phases, it is suggested that the decisive factor for forming the critical nucleus isn't a but the entropy offusion. According to this idea, the entropy-undercooling regime criterion for metastable phase formation was reviewed by using REFeO3 (RE: Rare-earth element) as the model material.
