[S-MP23] Physics and Chemistry of Minerals

[SMP23-05] Investigation of tetrahedral symmetry structure in SiO₂ glass under pressure by in situ pair distribution function measurement

★Invited Papers

*Yoshio Kono¹, Koji Ohara², Nozomi Kondo¹, Hiroki Yamada², Satoshi Hiroi², Fumiya Noritake³, Kiyofumi Nitta², Oki Sekizawa², Yuji Higo², Yoshinori Tange², Hirokatsu Yumoto^2,4, Takahisa Koyama^2,4, Hiroshi Yamazaki^2,4, Yasunori Senba^2,4, Haruhiko Ohashi^2,4, Shunji Goto^2,4, Ichiro Inoue⁴, Yujiro Hayashi⁴, Kenji Tamasaku⁴, Taito Osaka⁴, Jumpei Yamada⁴, Makina Yabashi⁴ (1.Geodynamics Research Center, Ehime University, 2.SPring-8, JASRI, 3.University of Yamanashi, 4.RIKEN SPring-8 Center)

Keywords:Magma, High pressure, SiO2 glass

Understanding the structural origin of the anomalous properties in SiO₂ and silicate melts/glasses at high pressure conditions is fundamental in understanding the nature and dynamics of silicate magmas in the Earth and planet. Theoretical studies of SiO₂ liquid have suggested that the second shell structure of silicon is the key to understanding the anomalous properties of SiO₂ liquid at high temperatures and high pressures [Shi and Tanaka, 2018, PNAS, 115, 1980-1985]. However, due to experimental difficulties of accurate measurement of the structure of amrphous materials at in situ high pressure condition, there has been no experimental observation of the structure of the silicon’s second shell in SiO₂ liquid and/or glass at in situ high pressure and/or high temperature conditions. In order to overcome the technical difficulty, we developed in situ high-pressure pair distribution function measurement by utilizing high flux and high energy X-rays from undulator sources at BL05XU and BL37XU beamlines in SPring-8. Our newly developed experimental setup enable us to measure structure factor [S(Q)] of SiO₂ glass at wide range of Q to 19-20 Å^-1 under in situ high pressure conditions up to 6.0 GPa. By combining the high-pressure experimental S(Q) precisely determined by using monochromatic X-ray with the MD (molecular dynamics simulation)-RMC (reverse Monte Carlo) modelling, we are able to investigate detailed structural behavior of SiO₂ glass beyond the nearest neighbor distances under in situ high pressure conditions [Kono et al., 2022, Nature Communications, 13, 2292, https://www.nature.com/articles/s41467-022-30028-w]. We find bimodal features in the translational order of the silicon’s second shell in terms of the structural parameter z and in the void radius formed from silicon atoms in SiO₂ glass under pressure. The bimodal behavior in the distribution of the parameter z observed in SiO₂ glass with varying pressure in this study is consistent with that simulated in SiO₂ liquid with varying temperature in theoretical study. At low pressure conditions, SiO₂ glass has tetrahedral symmetry structure with separation between the first and second shells of silicon, which corresponds to the S state structure reported in theoretical study of 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.