Phase relations in δ-AlOOH investigated with ab-initio calculations

As a high-pressure phase of diaspore and boehmite, δ-AlOOH is a crucial hydrous phase that can transport water in subducted slabs to the Earth's interior. Knowledge of phase relations in AlOOH is vital to understanding the state of water and the mechanisms of water transportation to the deep mantle. Most previous theoretical studies focused on the low-pressure static behavior of δ-AlOOH, leaving not only issues such as hydrogen-bond symmetrization under pressure unresolved but also high-temperature high-pressure phase boundaries not fully discussed. Recent experiments [1] show that the stability field of δ-AlOOH covers the entire pressure range of the Earth's lower mantle and this phase could remain stable in colder slabs down to the core-mantle boundary.
We present an ab initio study of the structural and thermodynamic stability of δ-AlOOH and its related polymorphs to multi-Mbar pressures. We address hydrogen-bond symmetrization in AlOOH and the phase boundaries between its well-known phases. Moreover, we are also interested in assessing the dehydration boundary up to pressures exceeding those of the Earth's mantle as well as the possibility of alternative polymorphs at pressures relevant to terrestrial exoplanets.

[1] Y. Duan et al., Earth & Planet. Sci. Lett. 494, 92–98 (2018).