Structural ordering in a calcium silicate glass

Abstract

Glasses, and the liquids from which they are generally quenched, conspicuously lack translational periodicity and from this experimental fact it is a short step to assume that glasses are essentially disordered. In silicate glasses, the strongly covalently bonded silicon and oxygen atoms are thought to form a network structure within which ionically bonded 'network-modifying' atoms such as Li, Na, Ca, Mg and Fe substitute randomly in suitable interstices. Taken to extremes, this view leads to a structure in which any local order is restricted to the environment of the strongly bound Si atoms, with ions like Ca²⁺ having a highly distorted and variable first coordination shell of oxygens. The evidence for local ordering around the Si is overwhelming; in vitreous silica, SiO₄ tetrahedra are only a little more distorted than in the crystal. Here we present conclusive experimental evidence, from neutron scattering with isotopic substitution, that Ca, a typical alkaline-earth 'modifier', also has a comparatively well ordered first-neighbour shell in a silicate glass, with indications of ordering beyond that.