Five-fold local symmetries in metallic liquids and glasses

In this article, we review the experimental, theoretical and simulation studies on five-fold local symmetries in metallic liquids and glasses. In the early study on simple liquid structure, it has been realized that five-fold local symmetry plays a key role in irregular structures, supercooling and crystallization of simple liquids. In particular, icosahedral short-range order, representative of five-fold local symmetry, has attracted much attention. In addition, researches proposed a dense random packing model for simple liquid structure in 1959, and found a wide variety of polyhedra and absolute predominance of pentagonal faces in simple liquids, and also pointed out that pentagonal arrangements can only occur in very complex structures such as some of the alloy structures. Based on the Frank's hypothesis of icosahedral short-range order as blocking unit in a simple liquid, a lot of theoretical and experimental efforts have been made to confirm its existence in simple liquids, metallic liquids and glasses. So far, several theoretical methods have been developed for characterizing local atomic structures in simple liquids, such as bond-orientational order parameter, Honeycutt-Andersen index, and Voronoi tessellation. Although the local atomic symmetries in atomic structures in metallic liquids and glasses can be characterized by these methods and the geometries of the atomic structures in liquids and glasses have received much more attention, an atomic cluster model has been developed for establishing the structure-property relationship in metallic liquid and glass. Due to the diversity of the atomic clusters in both type and population of different metallic liquids and glasses, the atomic cluster model could not present a simple description of structure-property relationship. Based on the fundamental characteristics of metallic liquids and glasses, five-fold local symmetry, the structure-property relationship in metallic liquids and glasses, such as dynamic crossover, glass transition, liquid-liquid phase transition, and deformation can be well described in simple, quantitative and unified ways, and therefore a clear physical picture can be provided. All these studies indicate that five-fold local symmetry as a structural parameter is simple, general and effective.