Effect of liquid structure on kinetic stability and glassy motifs.

Abstract

Although atoms in a supercooled liquid appear randomly arranged, local crystal-like structural motifs can emerge. Such motifs are assumed to reduce the kinetic stability of the liquid against crystallization. In this study, the relation between local structure and kinetic stability of a supercooled liquid alloy is investigated through molecular dynamics simulations, using a model alloy that forms Laves phases. To detect subtle structural differences relevant to crystallization, the liquid structure is analyzed using the p_{3} code. This method encodes each local environment as a graph with or without species labels. Unexpectedly, our analysis shows that liquids with more crystalline structural units can remain metastable for a longer time before crystallization. This apparent paradox is resolved by showing that kinetic stability increases when crystalline units do not easily connect to form extended clusters. Further analysis shows that clusters with the same Voronoi index can yield distinct graphs that exhibit different correlations with kinetic stability, revealing limitations of the traditional Voronoi index. Even clusters with the same graph can correlate differently due to variations in species labeling, highlighting the importance of chemical ordering. To capture medium-range order undetected by conventional analysis based on crystalline units, the concept of loose-crystalline units is introduced. Analysis of loose-crystalline configurations suggests that the previously observed fivefold twin-like motifs in the glassy state are remnants of crystal seeds frozen before they could fully develop. Our approach applies to various systems that form Laves phases.