Ab initio investigation of H-bond disordering in $δ$-AlOOH

Abstract

δ-AlOOH (δ) is a high-pressure hydrous phase that participates in the deep geological water cycle. At 0 GPa, δ has asymmetric hydrogen bonds (H-bonds). Under pressure, it exhibits H-bond disordering, tunneling, and finally, H-bond symmetrization at ∼18 GPa. This study investigates these 300 K pressure-induced state changes in δ with ab initio calculations. H-bond disordering in δ was modeled using supercell multi-configuration quasiharmonic calculations. We examine: (a) energy barriers for proton jumps, (b) the pressure dependence of phonon frequencies, (c) 300 K compressibility, (d) neutron diffraction pattern anomalies, and (e) compare ab initio bond lengths with measured ones. Such thorough and systematic comparisons indicate that: (a) proton "disorder" has a restricted meaning when applied to δ. Nevertheless, H-bonds are disordered between 0 and 8 GPa, and a gradual change in H-bond configuration results in enhanced compressibility. (b) several structural and vibrational anomalies at 8 GPa are consistent with the disappearance of a particular (HOC-12) H-bond configuration and its change into another one (HOC-11*). (c) between 8-11 GPa, H-bond configuration (HOC-11*) is generally ordered, at least in short-to mid-range scale. (d) between 11.5-18 GPa, H-bond lengths approach a critical value that impedes compression, resulting in decreased compressibility. In this pressure range, especially approaching H-bond symmetrization at ∼18 GPa, anharmonicity and tunneling should play an essential role in the proton dynamics. Further simulations accounting for these effects are desirable to clarify the protons' state in this pressure range.

Ab initio calculations of third-order elastic coefficients

“…Given the close relationship between TOE and crystal anharmonic theory [7, Section 29.1], a better understanding of TOE theory allows us to better address thermoelasticity or thermal expansivity [24,25] at finite pressure for highly-anharmonic (e.g., [26]) or highly-anisotropic solids (e.g., serpentine).…”