Densification mechanisms of oxide glasses and melts

We consider the structural response of oxide glasses and melts to high-pressure conditions, and focus on the transformations that take place to the network forming tetrahedral AO4 or planar triangular AO3 motifs (A = Si, Ge, or B) of the ambient pressure materials. The oxygen-packing fraction ηO is an important indicator of when these structural changes are likely to occur. Densification occurs in stages, where a plateau of stability for a given type of network-forming motif is followed by a regime of change. The conversion of tetrahedral AO4 to octahedral AO6 motifs starts at ηO = 0.58, and fivefold coordinated AO5 units act as important intermediaries during this transformation. The position of the first sharp diffraction peak in measured diffraction patterns is sensitive to ηO, enabling it to be used as a marker for structural change. The transformation of AO6 to higher coordinated structural motifs starts when ηO = 0.72.