Hydrogen-Bond Symmetrization of \delta-AlOOH

The \delta-AlOOH can transport water into the deep mantle along cold subducting slab geotherm. We investigate the hydrogen-bond symmetrization behavior of \delta-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 \sim 10.0 GPa. The symmetrization pressure obtained by MD simulation is related to T by P\,(\rm GPa)=13.9\,(\rm GPa)+0.01\,(\rm GPa/K)\times T\,(\rm K). We conclude that \delta-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: K_0\,(\rm GPa)=181\,(\rm GPa)-0.013\,(\rm GPa/K)\times T\,(\rm K) for \delta-AlOOH with asymmetric hydrogen bond, and K_0\,(\rm GPa)=216\,(\rm GPa)-0.013\,(\rm GPa/K)\times T\,(\rm K) for \delta-AlOOH with symmetric hydrogen bond. Our results provide an important insight into the existent form and properties of \delta-AlOOH in the lower mantle.