Transformation from a network to a close-packed structure in the structural models of CaO–SiO<sub>2</sub> binary glasses

Structural models of CaO–SiO2 binary glasses across the entire composition range, including compositions beyond the glass-forming region, were generated using molecular dynamics simulations. The packing density increased with CaO content, reproducing the experimental values within the known glass-forming region. Local structure analysis around Si atoms revealed a systematic transition in Qn species, indicating progressive network depolymerization with increasing CaO content. The reduced atomic arrangement method was employed to determine and visualize glass structures with different compositions. At a CaO content exceeding 60 mol %, where the corner-sharing SiO4 network was completely depolymerized, oxide ions formed a nearly close-packed arrangement. This unique atomic arrangement was corroborated by the oxygen–oxygen partial pair distribution function. The formation of the nearly close-packed oxide ion arrangement in CaO-rich compositions was attributed to the transformation from a network structure owing to strong depolymerization. The highly ordered atomic arrangement found in oxide glasses can be considered a type of hyperordered structure within disordered materials—an alternative to the conventional random network structure in oxide glasses.