X-ray and Neutron Study on the Structure of Hydrous SiO2 Glass up to 10 GPa

Experimental evidence of tetrahedral symmetry breaking in SiO2 glass under pressure

Bimodal behavior in the translational order of silicon's second shell in SiO₂ liquid at high temperatures and high pressures has been recognized in theoretical studies, and the fraction of the S state with high tetrahedrality is considered as structural origin of the anomalous properties. However, it has not been well identified in experiment. Here we show experimental evidence of a bimodal behavior in the translational order of silicon's second shell in SiO₂ glass under pressure. SiO₂ glass shows tetrahedral symmetry structure with separation between the first and second shells of silicon at low pressures, which corresponds to the S state structure reported in SiO₂ liquid. On the other hand, at high pressures, the silicon's second shell collapses onto the first shell, and more silicon atoms locate in the first shell. These observations indicate breaking of local tetrahedral symmetry in SiO₂ glass under pressure, as well as SiO₂ liquid.

Hard x-ray methods for studying the structure of amorphous thin films and bulk glassy oxides

High-energy photon diffraction minimizes many of the corrections associated with laboratory x-ray diffractometers, and enables structure factor measurements to be made over a wide range of momentum transfers. The method edges us closer toward an ideal experiment, in which coordination numbers can be extracted without knowledge of the sample density. Three case studies are presented that demonstrate new hard x-ray methods for studying the structure of glassy and amorphous materials. First, the methodology and analysis of high-energy grazing incidence on thin films is discussed for the case of amorphous In₂O₃. The connectivity of irregular InO₆polyhedra are shown to exist in face-, edge- and corner-shared configurations in the approximate ratio of 1:2:3. Secondly, the technique of high-energy small and wide angle scattering has been carried out on laser heated and aerodynamically levitated samples of silica-rich barium silicate (20BaO:80SiO₂), from the single phase melt at 1500^oC to the phase separated glass at room temperature. Based on Ba-O coordination numbers of 6 to 7, it is argued that the although the potential of Ba is ionic, it is weak enough to cause the liquid-liquid immiscibility to become metastable. Lastly, high-energy small and wide angle scattering has also been applied to high water content (up to 12 wt.%) samples of hydrous SiO₂glass quenched from 1500^oC at 4 GPa. An increase of Si₁-O₂correlations at 4.3 Å is found to be consistent with an increase in the population of three-membered SiO₄rings at the expense of larger rings.