R3: High-pressure science and deep Earth’s material

[R3-01] Pressure-induced polyamorphic transition in CaAl₂O₄ glass revealed by elastic wave velocity and X-ray diffraction measurements and molecular dynamics simulations

「招待講演」

*Itaru Ohira¹, Yoshio Kono^2,3, Steeve Gréaux³, James W E Drewitt⁴, Sandro Jahn⁵, Fumiya Noritake⁶, Koji Ohara^7,8, Satoshi Hiroi^7,8, Nozomi M Kondo⁹, Rostislav Hrubiak¹⁰, Yuji Higo⁸, Noriyoshi Tsujino⁸, Sho Kakizawa⁸, Kiyofumi Nitta⁸, Oki Sekizawa⁸ (1. Gakushuin Univ. Sci., 2. Kwansei Gakuin Univ. Sci., 3. Ehime Univ. GRC, 4. U. Bristol Phys., 5. U. Cologne IGM, 6. U. Yamanashi Interdisciplinary Research, 7. Shimane Univ. Materials for Energy, 8. JASRI, 9. Okayama Univ. IPM, 10. ANL HPCAT)

Keywords:oxide glass, polyamorphism, elastic wave velocity, synchrtron X-ray diffraction, molecular dynamics simulations

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 v_P and v_S 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).