Interstice correlation functions; a new, sensitive characterisation of non-crystalline packed structures

Abstract

A dense random packing of soft spheres is developed as a model of an “ideal” single component metallic glass. This structure can be considered as an assembly of distorted octahedra and tetrahedra, as can simple close-packed crystal structures. The more complex polyhedra (Bernal holes) previously identified in hard sphere models are largely squeezed out as a result of the additional free volume conferred by the soft potential. The local correlations of these basic tetrahedral and octahedral building blocks are discussed as possible characterisations of non-crystalline packed structures, together with their organisation around the component atoms. These characterisations of local correlations seem more sensitive and more reliably interpreted than the Voronoi polyhedra, particularly in detecting local icosahedral structures. Extensions of the descriptive techniques developed include examinations of weighted interstice connectivity, using data on the sizes of the interconnecting largely triangular “necks” as weighting factors. Departures from this idealised framework of linked tetrahedra and octahedra can be considered as defects, or distortions from the idealised structure, and the relative populations of local groupings provide a possible route to configurational entropy calculations.