Formation and Structure of a Dense Octahedral Glass

We have performed insitu x-ray and neutron-diffraction measurements, and molecular dynamics simulations, of GeO₂, an archetypal network-forming glass under pressure. Below 5GPa, additional atoms encroaching on the first tetrahedral shell are seen to be a precursor of local coordination change. Between 6 and 10GPa, we observe structures with a constant average coordination of ∼5, indicating a new metastable, intermediate form of the glass. At 15GPa, the structure of a fully octahedral glass has been measured. This structure is not retained upon decompression and, therefore, must be studied insitu.