Similarities in amorphous and crystalline transition metal–metalloid alloy structures

Abstract

In many amorphous and glassy systems the local structure seems to be essentially the same in amorphous and crystalline phases. It thus becomes possible to define a ‘local structural unit’, such as the SiO₄ group in silicates. Determination of the structure then reduces to the problem of describing how the local units are connected and packed in the medium-range structure. There is mounting evidence that alloys of transition metals with metalloids are more adequately described by arrangements of stereo-chemically-defined structural units than by alternative descriptions in terms of random packing of atoms. There are three phases, Fe₃C, Ti₃P and Fe₃P, which may serve as models of local structural units. These lattices differ to a small degree in the symmetry of the metalloid nearest-neighbour shell but there are major differences in the arrangement of local structural units. Although the experimental evidence is limited, it seems that the distinctive features of the local and medium-range topologies of the crystalline phases are represented in the structures of each of the corresponding glasses.