Elastic softening driven by hydrogen disorder in δ-(Al,Fe)OOH

Abstract:

Subduction of hydrous minerals controls the delivery of water in the Earth’s interior. Characterizing the elastic properties of candidate hydrous minerals and their evolution with pressure would provide tighter constraints on the quantification and distribution of water in the Earth’s interior. Members of the solid solution between phase H, δ-AlOOH and ε-FeOOH are thought to be the main carriers of water in the lower mantle. End-members of this solid solution have been studied using theoretical and experimental approaches. Previous studies show pressure drives a change in the structure of δ-AlOOH, causing the hydrogen bonds to go from an asymmetric to a disordered configuration, triggering a transition from P2₁nm to the Pnnm space group. Theoretical predictions suggest that this transition may affect the elastic response of this oxy-hydroxide. Previous studies, limited to X-ray and neutron diffraction, hamper an accurate description of the effect of this phase transition on the elastic tensor (C_ij), and limit our understating of the relation between hydrogen bond evolution and elastic properties.

In this study, single crystals of Fe-bearing δ-AlOOH with 3% of Fe were synthesized in a multi-anvil press. Two oriented single crystals were double-sided polished and loaded in the same diamond-anvil cell. Simultaneous X-ray diffraction and Brillouin scattering measurements were performed to constrain unit-cell parameters and elastic tensors in a pressure range relevant to the P2₁nm – Pnnm transition. Our results provide experimental constrains on the elastic tensor of both P2₁nm and Pnnm structures of δ-(Al,Fe)OOH. Results also show that C₂₂, C₁₂ and C₂₃ soften while approaching the transition pressure and rapidly increase after transformation. This behavior is reflected on the bulk modulus, while the shear modulus shows a monotonic increase with pressure. Our results show that an elastic softening marks the evolution from an asymmetric to a disordered configuration of the hydrogen bonds in δ-(Al,Fe)OOH, suggesting that such behavior could be a precursor of the symmetrisation. Our findings should provide important implications for the prediction of the elastic properties of hydrous phases that are expected to experience hydrogen bond symmetrisation with pressure, such as H₂O ice and phase D.