Hydrogen-Bond Symmetrization of δ-AlOOH**Supported by the National Natural Science Foundation of China under Grant Nos U1232204, 41473056, 11422431 and 11634001, the National Key Research and Development Program of China under Grant No 2016YFA0300901, and the National Basic Research Program of China under Grant No 2013CB934600.

Abstract

The δ-AlOOH can transport water into the deep mantle along cold subducting slab geotherm. We investigate the hydrogen-bond symmetrization behavior of δ-AlOOH under the relevant pressure-temperature condition of the lower mantle using ab initio molecular dynamics (MD). The static symmetrization pressure of 30.0 GPa can be reduced to 17.0 GPa at 300 K by finite-temperature (T) statistics, closer to the experimental observation of ∼10.0 GPa. The symmetrization pressure obtained by MD simulation is related to T by P (GPa) + 13.9 (GPa) = 0.01 (GPa/K) × T (K). We conclude that δ-AlOOH in the lower mantle exists with symmetric hydrogen bond from its birthplace, or someplace slightly deeper, to the core-mantle boundary (CMB) along cold subducting slab geotherm. The bulk modulus decreases with T and increases anomalously upon symmetrization: for δ-AlOOH with asymmetric hydrogen bond, and for δ-AlOOH with symmetric hydrogen bond. Our results provide an important insight into the existent form and properties of δ-AlOOH in the lower mantle.