Entropy-Undercooling Regime Criterion for Metastable Phase Formation in Oxide Material

Abstract

Undercooling a melt often facilitates a metestable phase to preferentially nucleate. In the present study, the formation of a metastable phase from undercooled melts was investigated 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 γ. In fact, on the simple liquid such as the melt of a mono-atomic metal, Spaepen’s negentropic model regarding γ suggested the scaling factor α between γ and the entropy of fusion 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 α 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 α but the entropy of fusion. According to this idea, we proposed the entropy-undercooling regime criterion for metastable phase formation and, using REFeO3 (RE: Rare-earth element) as the model material, experimentally verified the validity of the criterion.