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...
The high-pressure structural and vibrational properties of Bi2S3 have been probed up to 65 GPa with a combination of experimental and theoretical methods. The ambient-pressure Pnma structure is found to persist up to 50 GPa; further compression leads to structural disorder. Closer inspection of our structural and Raman spectroscopic results reveals notable compressibility changes in specific structural parameters of the Pnma phase beyond 4-6 GPa. By taking the available literature into account, we speculate that a second-order isostructural transition is realized near that pressure, originating probably from a topological modification of the Bi2S3 electronic structure near that pressure. Finally, the Bi(3+) lone-electron pair (LEP) stereochemical activity decreases against pressure increase; an utter vanishing, however, is not expected until 1 Mbar. This persistence of the Bi(3+) LEP activity in Bi2S3 can explain the absence of any structural transitions toward higher crystalline symmetries in the investigated pressure range.
Hydrogen is likely one of the light elements in the Earth’s core. Despite its importance, no direct observation has been made of hydrogen in an iron lattice at high pressure. We made the first direct determination of site occupancy and volume of interstitial hydrogen in a face-centered cubic (fcc) iron lattice up to 12 GPa and 1200 K using the in situ neutron diffraction method. The transition temperatures from the body-centered cubic and the double-hexagonal close-packed phases to the fcc phase were higher than reported previously. At pressures <5 GPa, the hydrogen content in the fcc iron hydride lattice ( x ) was small at x < 0.3, but increased to x > 0.8 with increasing pressure. Hydrogen atoms occupy both octahedral (O) and tetrahedral (T) sites; typically 0.870(±0.047) in O-sites and 0.057(±0.035) in T-sites at 12 GPa and 1200 K. The fcc lattice expanded approximately linearly at a rate of 2.22(±0.36) Å 3 per hydrogen atom, which is higher than previously estimated (1.9 Å 3 /H). The lattice expansion by hydrogen dissolution was negligibly dependent on pressure. The large lattice expansion by interstitial hydrogen reduced the estimated hydrogen content in the Earth’s core that accounted for the density deficit of the core. The revised analyses indicate that whole core may contain hydrogen of 80(±31) times of the ocean mass with 79(±30) and 0.8(±0.3) ocean mass for the outer and inner cores, respectively.
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