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

Abstract

In this paper, we report that $\mathrm{Ca}{(\mathrm{OH})}_{2}$, 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 $P2{}_{1}/c$ and $Pnma$ symmetry are proposed at high pressure. The monoclinic $P2{}_{1}/c$ structure is the most stable phase after 23 GPa and will undergo a phase transition into a $Pnma$ 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 $\mathrm{Ca}{(\mathrm{OH})}_{2}$ that we proposed are stable against decomposition into CaO and ${\mathrm{H}}_{2}\mathrm{O}$ at the conditions corresponding to lower mantle geotherms. Our results shed light on the possible presence of $\mathrm{Ca}{(\mathrm{OH})}_{2}$ in the Earth's mantle as well as its water transportation in the Earth's interior.