The atoms in liquids and glasses are ordered on nearest-neighbour length scales but, unlike crystalline materials, they are not ordered over much longer distances. Glasses are formed when a liquid is quenched so quickly that the atoms do not have time to re-arrange themselves into a crystalline state, which has a lower energy, and they form a disordered amorphous network instead.

This disorder makes liquids and glasses notoriously difficult to study but Salmon and co-workers have now overcome some of these problems by using neutron diffraction to probe samples of zinc chloride and germanium selenide in which various atoms have been replaced by different isotopes of the same element. This approach exploits the fact that different isotopes scatter neutrons by different amounts but the isotopic substitution does not change the structure of the material. This allows the team to determine the relative positions of pairs of atoms in the two materials.

Salmon and colleagues found that the ordering of atomic pairs in the two structures was very similar on both intermediate (about 6 Angstroms) and extended (around 60 Angstroms) scales. This is surprising because the two materials have very different chemical properties: the bonds in the zinc chloride network are ionic, while germanium selenide is a covalently bonded material. According to the Bath-Bristol-ILL team the results suggest that the structural ordering they observe could be a general feature of all glasses.

"Our results provide insights into the nature of glassy networks and may also lead to the preparation of new materials by rational design," Salmon told PhysicsWeb. "These materials include 'optically active' glasses for applications such as fibre lasers and amplifiers, and glasses used for the storage of nuclear waste."