10:00 〜 10:15
[R3-01] 弾性波速度および放射光X線回折測定と分子動力学シミュレーションによるCaAl2O4ガラスの圧力誘起ポリアモルフィック転移の解明
「招待講演」
キーワード：酸化物ガラス、ポリアモルフィズム、弾性波速度、放射光X線回折、分子動力学シミュレーション
In recent years, there has been increasing emphasis on the fabrication and characterization of non-conventional network glass formers such as CaO–Al2O3, BaO–Al2O3, and BaO–TiO2 in material science [e.g., 1]. Although these systems are different from those of magmas traditionally studied in geoscience field, recent experiments have confirmed the formation of network-modifiers- and Al-rich partial melts at the conditions of the transition zone and the lower mantle [2,3]. Therefore, the high-pressure studies of non-conventional network glass formers, as well as typical network-forming oxide glasses such as SiO2 glass, are also important in understanding the pressure-induced changes of structure and physical properties of the magmas in the deep earth.
In this study, we conducted elastic wave velocity and XRD measurements and molecular dynamics (MD) simulations on CaAl2O4 glass. Ultrasonic velocity measurements up to 24 GPa at BL04B1 at SPring-8 reveal abrupt and irreversible increases in the vP and vS at ~8–10 GPa. Total structure factor and pair distribution functions measured by synchrotron XRD at 16-BM-B at APS and BL37XU at SPring-8 show a rapid change in the intermediate range structure, which is likely attributed to a rearrangement of Ca ions over this narrow pressure condition. Structure models obtained from MD simulations reveal that this intermediate range structure is explained by a transition of Ca–O void radius distribution from a bimodal distribution with peaks at ~2.1 Å and ~2.4 Å to a single distribution centered at ~2.1 Å. The abrupt structural changes involving the rapid increase in elastic wave velocity in CaAl2O4 glass are markedly different to the continuous transformations reported in SiO2 glass. The polyamorphic transition observed in this study may be one of the key mechanisms in the densification and the changes in seismic wave velocity of the magma just above the 410 km discontinuity.
References
[1] A. Masuno, J. Phys. Soc. Jpn. 91, 091003 (2022).
[2] G. K. Pradhan et al., Earth Planet. Sci. Lett., 431, 247 (2015).
[3] A. Nakajima et al. Sci. Rep. 9, 7420 (2019).
In this study, we conducted elastic wave velocity and XRD measurements and molecular dynamics (MD) simulations on CaAl2O4 glass. Ultrasonic velocity measurements up to 24 GPa at BL04B1 at SPring-8 reveal abrupt and irreversible increases in the vP and vS at ~8–10 GPa. Total structure factor and pair distribution functions measured by synchrotron XRD at 16-BM-B at APS and BL37XU at SPring-8 show a rapid change in the intermediate range structure, which is likely attributed to a rearrangement of Ca ions over this narrow pressure condition. Structure models obtained from MD simulations reveal that this intermediate range structure is explained by a transition of Ca–O void radius distribution from a bimodal distribution with peaks at ~2.1 Å and ~2.4 Å to a single distribution centered at ~2.1 Å. The abrupt structural changes involving the rapid increase in elastic wave velocity in CaAl2O4 glass are markedly different to the continuous transformations reported in SiO2 glass. The polyamorphic transition observed in this study may be one of the key mechanisms in the densification and the changes in seismic wave velocity of the magma just above the 410 km discontinuity.
References
[1] A. Masuno, J. Phys. Soc. Jpn. 91, 091003 (2022).
[2] G. K. Pradhan et al., Earth Planet. Sci. Lett., 431, 247 (2015).
[3] A. Nakajima et al. Sci. Rep. 9, 7420 (2019).