Ab initio investigation of hydrogen-bond disorder in δ-AlOOH*

δ-AlOOH (δ) is an important carrier of water in the lower mantle. Attempts to resolve the structure of δ revealed that hydrogen bonds (H-bond) “symmetrize” under compression in a process similar to the high-pressure ice VII- or ice VIII-X transition. Like H₂O-ice, δ has its own “ice-disorder rules” and disorder is also observed experimentally prior to H-bond symmetrization [1]. H-bond disorder in δ had been suggested by an ab initio study as a possibility based on its broad band of Raman active OH-stretching modes [2].

In this ab initio study, we address the H-bond disordering process that precedes its symmetrization from several perspectives, including a) the accuracy of standard DFT functionals to address the structural properties of δ, b) vibrational stability and the pressure-range of validity of the quasiharmonic approximation (QHA), and c) order-disorder transition using a multi-configuration QHA calculation at 300 K. With these results, we reproduce the structural behavior and the neutron diffraction peculiarities under pressure.

[1] A. Sano-Furukawa et al., DOI: 10.1038/s41598-018-33598-2
[2] J. Tsuchiya et al., DOI: 10.2138/am.2008.2627