Ultrahigh-pressure polyamorphism in GeO ₂ glass with coordination number >6

Abstract

Knowledge of pressure-induced structural changes in glasses is important in various scientific fields as well as in engineering and industry. However, polyamorphism in glasses under high pressure remains poorly understood because of experimental challenges. Here we report new experimental findings of ultrahigh-pressure polyamorphism in GeO 2 glass, investigated using a newly developed double-stage large-volume cell. The Ge-O coordination number (CN) is found to remain constant at ∼6 between 22.6 and 37.9 GPa. At higher pressures, CN begins to increase rapidly and reaches 7.4 at 91.7 GPa. This transformation begins when the oxygen-packing fraction in GeO 2 glass is close to the maximal dense-packing state (the Kepler conjecture = ∼0.74), which provides new insights into structural changes in network-forming glasses and liquids with CN higher than 6 at ultrahigh-pressure conditions. high pressure | polyamorphism | glass | oxygen packing U nderstanding the structural response of network-forming glasses to pressure is of great interest not only in condensed matter physics, geoscience, and materials science, but also in engineering and industry. As prototype network-forming glasses, silica (SiO 2 ) and germania (GeO 2 ) have been the most extensively studied (1-5). These two glasses have similar structural change pathways at high pressures. At ambient pressure, both glasses are composed of corner-linked AO 4 tetrahedra, with atom A (Si or Ge) in fourfold coordination (6). Under compression, the coordination gradually changes from 4 to 6 over a wide pressure range [∼15-40 GPa for SiO 2 glass (2, 4) and ∼5-15 GPa for GeO 2 glass (1, 3, 5)].A recent study (7) found that evolution of network-forming structural motifs in glasses and liquids at high pressures can be rationalized in terms of oxygen-packing fraction (OPF). Fourfoldcoordinated structural motifs in SiO 2 and GeO 2 glasses are stable over a wide range of OPF between 0.40 and ∼0.59. The fourfoldcoordinated structural motifs become unstable when the OPF approaches the limit of random loose packing of hard spheres (0.55-0.60) (8, 9). When OPF >∼0.60, coordination number (CN) gradually increases with OPF to the limit of random close packing (0.64) (8, 9), where CN increases sharply to 6 with almost-constant OPF ∼0.64. Higher-pressure data for SiO 2 glass suggest the existence of another stability plateau for sixfold-coordinated structural motifs, with OPF of up to ∼0.72 (7).The highest coordination that has been experimentally determined so far in SiO 2 and GeO 2 glasses is 6. X-ray diffraction measurement for SiO 2 glass confirmed that sixfold-coordination structural motifs are stable up to 100 GPa (4). For GeO 2 glass, X-ray and neutron diffraction data are limited to 18 GPa (1, 3, 5). X-ray absorption spectroscopic measurements were conducted to 64 GPa (10, 11). Ref. 11 showed no major change in X-ray absorption fine structure up to 64 GPa, although a slight discontinuous change in density is observed around 40-45 GPa.Some simulation studies predicted the existe...

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