Elastic properties and short-to medium-range order in glasses

Very different materials are named "Glass," with Young's modulus (E) and Poisson's ratio (nu) extending from 5 to 180 GPa and from 0.1 to 0.4, respectively, in the case of bulk inorganic glasses. Although glasses have in common the lack of long-range order in the atomic organization, they offer a wide range of structural features at the nanoscale and we show in this analysis that beside the essential role of elastic properties for materials selection in mechanical design, the elastic characteristics (E, nu) at the continuum scale allow to get insight into the short- and medium-range orders existing in glasses. In particular, nu, the atomic packing density (C-g) and the glass network dimensionality appear to be strongly correlated. Maximum values for nu and C-g are observed for metallic glasses (nu similar to 0.4 and C-g > 0.7), which are based on cluster-like structural units. Atomic networks consisting primarily of chains and layers units (chalcogenides, low Si-content silicate, and phosphate glasses) correspond to nu > 0.25 and C-g > 0.56. On the contrary, nu < 0.25 is associated with a highly cross-linked network, such as in a-SiO2, with a tri-dimensional organization resulting in a low packing density. Moreover, the temperature dependence of the elastic moduli brings a new light on the structural changes occurring above the glass transition temperature and on the depolymerization rate in the supercooled liquid. The softening rate depends on the level of cooperativity of atomic movements at the source of the deformation process, with an obvious correlation with the "fragility" of the liquid.