[S-IT17] Property and role of liquids inside terrestrial planets

[SIT17-P05] Structural study of silica-free calcium aluminate glass at high-pressures using the synchrotron X-ray facilities

*Itaru Ohira¹, Yoshio Kono², Steeve Gréaux², Nozomi M. Kondo², Rostislav Hrubiak³, Yuji Higo⁴ (1.Department of Chemistry, Faculty of Science, Gakushuin University, 2.Ehime University, 3.Argonne National Laboratory, 4.Japan Synchrotron Radiation Research Institute)

Aluminum ion (Al3+) is considered to play an important role in densification of oxide melts and glasses, structural analogs of melts, at high-pressure conditions. To understand the behaviors of Al3+ in aluminosilicate glasses at high-pressure without contamination of those of Si4+, silica-free CaAl2O4 glass has been measured by using high-pressure in situ Raman spectroscopy [1] and XRD [2]. The Raman spectroscopic study found the change of spectral pattern at ~8 GPa, which is likely interpreted as the changes of intermediate- and/or short-range order(s) [1]. The XRD study reported the structural data (structure factor, S(Q), and pair distribution functions (PDF), g(r)) up to 30 GPa [2]. However, the XRD measurements were conducted with 5 GPa step, and the detail of structural change at ~8 GPa was not discussed.
To address the pressure-induced structure changes of CaAl2O4 glass, we conducted high-pressure in situ XRD and ultrasonic measurements. Spherical CaAl2O4 glass samples with a diameter of ~2 mm were synthesized by a container-less laser heating system at Tohoku University. High-pressure multi-angle energy-dispersive XRD measurements using a Paris-Edinburgh press (model VX-3) were conducted at the beamline 16-BM-B, Advanced Photon Source. From the XRD experiments, we obtained the S(Q) and g(r) of CaAl2O4 glass up to 10 GPa. We also performed ultrasonic measurements using the Kawai-type multi-anvil press apparatus (SPEED-1500) at the beamline BL04B1, SPring-8, and determined the elastic wave velocity (vP and vS) of CaAl2O4 glass up to 24 GPa.
The S(Q) of CaAl2O4 glass showed that the 1st peak (FSDP) shifted to higher Q and the intensity of 2nd peak started to increase above ~7 GPa. The g(r) determined from the S(Q) showed the onset of modest increase of Al–O bond length between 6–7 GPa. We also observed that the peak of Ca–Al correlation at ~3.6 Å approached to the shorter Al–Al correlation (~3.1 Å) with decreasing its intensity and finally disappeared at the pressure range from ~7 GPa to ~9 GPa. While the signs of structural changes obtained from the XRD measurements were not obvious, the elastic wave velocity measurement clearly demonstrated that the vP and vS of CaAl2O4 glass rapidly increased at 7–10 GPa. Then, the vP and vS monotonically increased up to 24 GPa, implying the Al–O coordination number (CN) increased toward 6. The results of in situ XRD and elastic wave velocity measurements suggest that CaAl2O4 glass undergoes the change of intermediate-range structure related to Al–Al and Ca–Al correlations between ~7–10 GPa, and this structural change may trigger the onset of Al–O CN increase from 4 to 6.

Reference
[1] Daniel et al. Chem. Geol. 128, 5–15 (1996).
[2] Drewitt et al. J. Phys.: Condens. Matter 27, 105103 (2015).