Stability of Ca ( OH ) 2 at Earth's deep lower mantle conditions

In this paper, we report that Ca (OH)2, portlandite, is predicted to be stable up to 100 GPa via an unbiased structural search method combined with first-principles calculations. Two pressure-induced structures with P21 / c and P n m a symmetry are proposed at high pressure. The monoclinic P21 / c structure is the most stable phase after 23 GPa and will undergo a phase transition into a P n m a phase at 78 GPa and 0 K. This phase transition is computed to occur at lower pressure as the temperature increases. The high-pressure phases of Ca (OH)2 that we proposed are stable against decomposition into CaO and H2O at the conditions corresponding to lower mantle geotherms. Our results shed light on the possible presence of Ca (OH)2 in the Earth's mantle as well as its water transportation in the Earth's interior.