Elastic anomalies across the P21nm-Pnnm structural phase transition in δ-(Al,Fe)OOH

Satta, N., Criniti, G., Kurnosov, A., Boffa Ballaran, T., Ishii T., & Marquardt, H.

Abstract

Hydrogen may be recycled into the Earth’s lower mantle by subduction and stabilized in solid solutions between phase H (MgSiO 4 H 2 ), δ-AlOOH, ε-FeOOH, and SiO 2 post-stishovite. In high-pressure oxyhydroxide phases, hydrogen is incorporated following the typical (OHO) sequence, adopting the asymmetric configuration O-H⋯O that evolves into a symmetric disordered state upon compression. Moreover, these iron-bearing aluminum oxyhydroxides [δ-(Al,Fe)OOH] present a structural phase transition from P 2 1 nm to Pnnm as pressure increases. Here, the single-crystal elasticity of the P 2 1 nm phase of δ-(Al 0.97 Fe 0.03 )OOH has been experimentally determined across the P 2 1 nm → Pnnm transition up to 7.94(2) GPa by simultaneous single-crystal X-ray diffraction (XRD) and Brillouin scattering at high pressures. The transition appears to be continuous, and it can be described with a second-, fourth-, and sixth-order terms Landau potential. Our results reveal an enhanced unit-cell volume compressibility, which is linked to an increase of the b - and a -axes linear compressibility in the P 2 1 nm phase of δ-(Al 0.97 Fe 0.03 )OOH prior to the transition. In addition, we observed the presence of elastic softening in the P 2 1 nm phase that mostly impacts the elastic stiffness coefficients c 12 , c 22 , and c 23 . The observed elastic anomalies cause a significant change in the pressure dependence of the adiabatic bulk modulus ( K S ). These results provide a better understanding of the relation between elasticity, P 2 1 nm → Pnnm structural phase transition, and hydrogen dynamics in δ-(Al 0.97 Fe 0.03 )OOH, which may be applied to other O-H⋯O-bettring phases.