Structural Study of δ-AlOOH Up to 29 GPa

Abstract

A structure and equation of the state of δ-AlOOH has been studied at room temperature, up to 29.35 GPa, by means of single crystal X-ray diffraction in a diamond anvil cell using synchrotron radiation. Above ~10 GPa, we observed a phase transition with symmetry changes from P21nm to Pnnm. Pressure-volume data were fitted with the second order Birch-Murnaghan equation of state and showed that, at the phase transition, the bulk modulus (K0) of the calculated wrt 0 pressure increases from 142(5) to 216(5) GPa.

Raman scattering and Cr3+ luminescence study on the structural behavior of δ-AlOOH at high pressures

“…The R 2 -R 1 separation decreases continuously with pressure, implying that the AlO 6 octahedron becomes less distorted under compression. Sano-Furukawa et al 2018;Simonova et al 2020). As shown in the inset of Figure 9 (a), the QE value, which is 1.0071 at ambient conditions, drops to 1.0033 at ~8 GPa after the phase transition from the P2 1 nm to the Pnnm structure, indicating that pressure-induced suppression of distortion in the AlO 6 octahedron occurs during the phase transition and further verifying further the conclusion inferred from R-lines splitting.…”

“…The frequencies of the vibrational modes are plotted as a function of pressure in Figure 5, together with the previous data of Mashino et al (2016). Clearly, the B 2 modes at 390 cm -1 and 416 cm -1 show abrupt changes in their pressure dependences at ~8 GPa, indicating the P2 1 nm-to-Pnnm phase transition observed in previous diffraction studies (Sano-Furukawa et al 2018;Sano-Furukawa et al 2009;Simonova et al 2020). In the pressure range of 0-8 GPa, these two modes increase rapidly with pressure and display nonlinear behavior.…”

“…A recent neutron diffraction study further revealed that this structural transition is accompanied with the order-to-disorder transformation of hydrogen along hydrogen bonds as a result of the tunneling effect of hydrogen, and the high-pressure disordered phase further transitions into an ordered phase with symmetrized hydrogen bonds or fully-centered phase at ~18 GPa without changing the Pnnm structure; in this ordered phase, hydrogen atoms are located at the midpoints of two neighboring hydrogen-bonded oxygen atoms (Sano-Furukawa et al 2018). However, the transition to the ordered symmetrization phase was not observed to relate to any unusual feature in previous X-ray diffraction or spectroscopic works (Kagi et al 2010;Simonova et al 2020). The structures of the disordered and ordered Pnnm phases are shown in Figure 1 (b) and Figure 1 (c), respectively.…”

“…http://www.minsocam.org/ depths in subduction zones, the structural behavior of δ-AlOOH at high pressure has attracted much attention. X-ray diffraction studies observed a significant change in the axial ratios of a/c and b/c at ~10 GPa, which is likely to correspond to the structural transition from P2 1 nm to Pnnm (Kuribayashi et al 2014;Sano-Furukawa et al 2009;Simonova et al 2020). An analysis of the unit-cell volume versus pressure data showed that the bulk modulus of the high-pressure Pnnm phase was ~52% higher than that of the low pressure P2 1 nm phase, indicating a stiffened unit cell after the phase transition (Simonova et al 2020).…”

“…X-ray diffraction studies observed a significant change in the axial ratios of a/c and b/c at ~10 GPa, which is likely to correspond to the structural transition from P2 1 nm to Pnnm (Kuribayashi et al 2014;Sano-Furukawa et al 2009;Simonova et al 2020). An analysis of the unit-cell volume versus pressure data showed that the bulk modulus of the high-pressure Pnnm phase was ~52% higher than that of the low pressure P2 1 nm phase, indicating a stiffened unit cell after the phase transition (Simonova et al 2020). Moreover, significant changes in the frequency slope with pressure for the vibrational modes related to hydrogen bonds were also indicated at ~10 GPa by IR spectroscopic measurements (Kagi et al 2010).…”