Formation of triclusters in silica melt under high pressure

Abstract

Silica (SiO₂) is the major glass-forming material, and the structures of silica glass and melt have been extensively studied using X-ray and neutron diffraction techniques. The diffraction data of silica glass and melt show the first sharp diffraction peak (FSDP) at Q ∼ 1.5 Å⁻¹, which is a signature of intermediate-range order. In this study, we performed classical molecular dynamics (MD) simulations at 2000 K and 5 GPa to understand the behaviour of the diffraction peak associated with the modification of intermediate-range order. The high-pressure melt data obtained show the decrease in the height of FSDP with a shift of the peak position to the high-Q side in X-ray diffraction data, although the average coordination number of four was maintained. In addition, we observed the formation of OSi₃ triclusters, which share an oxygen atom with a SiO₄ tetrahedron. This unusually dense packed atomic arrangement is the result of high pressure and is associated with the very sharp principal peak observed at Q ∼ 3 Å⁻¹ in the O–O partial structure factor derived by MD simulation.