52 Physical Science Research Frontiers 2022 Understanding the structural origin of the anomalous properties of tetrahedral liquids and amorphous materials at high pressure and/or high temperature conditions is of great interest in wide range of scientific fields. In particular, since SiO 2 is ubiquitous in the Earth, understanding the SiO 2 ʼs anomaly is fundamental not only in physics, but also in geophysics to understand nature of silicate magmas in the Earth and planet, and in materials science as a prototype network-forming glass. It has been reported that SiO 2 liquid shows anomalous density and compressibility behaviors at high temperatures and high pressures [1]. In addition, SiO 2 glass also shows compressibility maximum (bulk modulus minimum) at high pressure of ~2 – 3 GPa under room temperature condition [2]. Theoretical studies of SiO 2 liquid suggests that the second shell structure of silicon is the key to understanding the anomalous properties of SiO 2 liquid at high temperatures and high pressures. A structural parameter z ( z = d ji – d j’i , where d ji and d j’i is the distance from each silicon atom i to the fifth nearest silicon neighbor j and to the fourth nearest oxygen neighbor j’) was developed to investigate the second shell structure in SiO 2 liquid [1]. The theoretical study found a bimodal distribution in the structural parameter z with varying temperature, and the S and r states are assigned to the high and low distributions in the parameter z , respectively. The low-density S state in SiO 2 liquid consists of four silicon neighbor atoms in the first shell and exhibits high tetrahedral order with clear separation between the first and second shell. The fraction of the S state with high tetrahedrality is considered to be the controlling parameter of the anomalous properties of SiO 2 liquid at high temperatures and high pressures in theoretical study [1]. However, there has been no experimental observation of the structure of the silicon’s second shell in SiO 2 liquid and/or glass at in situ high pressure and/or high temperature conditions. In this work [3], we carried out in situ high- pressure pair distribution function measurement of SiO 2 glass by utilizing high flux and high energy X-rays from undulator sources at SPring-8 BL37XU and BL05XU (Fig. 1(a)). The structure of SiO 2 glass was measured at in situ high pressure conditions up to 6.0 GPa in a Paris-Edinburgh (PE) cell (Fig. 1(b)) by high-energy X-ray diffraction measurement with a collimation setup to obtain signals only from the SiO 2 glass sample. We obtained the Faber-Ziman structure factor, S ( Q ), of SiO 2 glass at the wide range of the momentum transfer Q up to 19 Å –1 at BL37XU and up to 20 Å –1 at BL05XU (Fig. 2), which is almost two times larger than that in conventional high-pressure angle-dispersive X-ray diffraction measurements. By combining the high-pressure experimental S ( Q ) precisely determined at a wide range of Q up to 19 – 20 Å –1 with the MD (molecular dynamics simulation)-RMC (reverse Monte Carlo) modeling, we were able to investigate in detail the structural behavior of SiO 2 glass beyond the nearest neighbor distances under in situ high pressure conditions. We found bimodal feature in the translational order of the silicon’s second shell in terms of the structural parameter z (Fig. 3(c)). The bimodal feature is consistent with that simulated in SiO 2 liquid with varying temperature in the theoretical study [1]. Structural origin of the anomalous properties of SiO 2 glass under pressure reveled by in situ high-pressure pair distribution function measurement Fig. 1. In situ high-pressure pair distribution function measurement at SPring-8 BL05XU (a) , and design of the Paris-Edinburgh (PE) cell assembly for the SiO 2 glass experiment up to 6.0 GPa under room temperature condition (b) . PE press X-ray Collimation slit Detector slit CdTe detector (a) Boron epoxy Polycarbonate ZrO 2 1 mm SiO 2 glass BN MgO (b) Ge detector 53 Research Frontiers 2022 The structure of SiO 2 glass with the characteristic distribution of the parameter z at 2.4 Å shows a tetrahedral symmetry structure formed from the nearest four silicon atoms in the first shell, and the first and second shells are clearly separated as the fifth neighbor silicon atom locates in the second shell (Fig. 3(b)). The structural feature corresponds to the low-density S state structure reported in the theoretical study of SiO 2 liquid [1]. On the other hand, the structure of SiO 2 glass with the characteristic distribution of z at 1.7 Å shows that the fifth neighbor silicon atom locates in the first shell ( Fig. 3(a)), which indicates collapse of the second shell onto the first shell and breaking of local tetrahedral symmetry. SiO 2 glass mainly consists of the low-density S state with tetrahedral symmetry structure at low pressures. On the other hand, the local tetrahedral symmetry structure breaks at high pressures, and the fraction of the S state in SiO 2 glass decreases under pressure, as well as theoretical observation in SiO 2 liquid at high temperatures and high pressures. The new experimental technique of high-pressure pair distribution function measurement particularly using the high-flux pink beam at BL05XU beamline opens new way to investigate structural behavior of liquids and amorphous materials at in situ high pressure and high temperature conditions. The PE cell enables us to conduct high-pressure experiments not only for glass but also for liquid up to 7 GPa and 2000°C [4]. These techniques would have wide ranging application not only in scientific fields such as physics, chemistry, geoscience, and materials science but also engineering and industry processes. Fig. 2. Structure factor [ S ( Q )] of SiO 2 glass measured at 0 GPa (orange) and 5.2 GPa (red). Black lines show S ( Q ) of the MD-RMC structure model derived based on the experimentally observed S ( Q ) at each pressure condition. S ( Q ) at 5.2 GPa is displayed by a vertical offset of +1.5. Fig. 3. The structural features in SiO 2 glass with the characteristic distribution of z = 2.4 Å (b) and z = 1.7 Å (a) . Translational order in SiO 2 glass as a function of the parameter z obtained in our experiment with MD-RMC modeling and MD simulation with BKS model at 0 and 5.2 GPa (c) . Yoshio Kono Geodynamics Research Center, Ehime University Email: kono.yoshio.rj@ehime-u.ac.jp References [1] R. Shi and H. Tanaka: Proc. Natl. Acad. Sci. USA 115 (2018) 1980. [2] A. Yokoyama et al. : J. Appl. Phys. 107 (2010) 123530. [3] Y. Kono, K. Ohara, N. M. Kondo, H. Yamada, S. Hiroi, F. Noritake, K. Nitta, O. Sekizawa, Y. Higo, Y. Tange, H. Yumoto, T. Koyama, H. Yamazaki, Y. Senba, H. Ohashi, S. Goto, I. Inoue, Y. Hayashi, K. Tamasaku, T. Osaka, J. Yamada and M. Yabashi: Nat. Commun. 13 (2022) 2292. [4] Y. Kono et al. : Phys. Earth. Planet. Inter. 228 (2014) 269. 5 2 0 15 10 0 Q (Å –1 ) S ( Q ) 0 1 2 3 5.2 GPa 0 GPa z (Å) Fraction 1.0 1.5 2.5 2.0 3.5 3.0 0.20 0.00 0.15 0.05 0.10 S state with tetrahedral symmetry structure at low pressure Breaking of tetrahedral symmetry at high pressure (a) (b) (c) Si atom Si atom O atom MD simulation (5.2 GPa) MD simulation (0 GPa) Experiment (5.2 GPa) Experiment (0 GPa) High pressure High pressure
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