Local and medium range structures in amorphous alloys

Abstract

Experimental evidence suggests that the local environment of certain atoms in amorphous alloys is well-defined. Thus the structures of transition metal-metalloid glasses appear to be based on a 9-atom polyhedron centred on the metalloid. There are some indications that this polyhedron (nearly always distorted) is best represented by a trigonal prism. There are no clear signs that directional bonds are important in defining the environment of, say, B in NiB alloys and isolated “molecular” trigonal prisms are not particularly more stable in a central force field than other candidate structures (e.g. Archimedean antiprisms). Both polyhedra are also relatively unstable unless they are fully capped. The existence of trigonal prismatic units in a-metals may thus be related to efficient, space-filling packing of atoms — in which trigonal prisms are a consequence of a “medium-range” organisational scheme. Several medium-range structures consisting of packed trigonal prisms, tetrahedra and octahedra are examined. These include structures based on crystals and non-periodic clusters with five-fold symmetry similar to the “curved space” structures proposed as models for amorphous semiconductors and oxide glasses. It is concluded that while details of the particular medium-range structure present in the glass cannot be established, as yet, a medium-range structure probably exists and represents a constrained subset of possible configurations. The implications for glass formation in transition metal-metalloid alloys are discussed and it is suggested that the composition-dependence of medium-range structural constraints are intimately involved.