Monoclinic high-pressure polymorph of AlOOH predicted from first principles

Aluminum oxide hydroxide, AlOOH, is a prototypical hydrous mineral in the geonomy. The study of the high-pressure phase evolution of AlOOH is of fundamental importance in helping to understand the role of hydrous minerals in the water storage and transport in Earth, as in other planets. Here, we have systematically investigated the high-pressure phase diagram of AlOOH up to 550 GPa using the efficient crystal structure analysis by particle swarm optimization (CALYPSO) algorithm in conjunction with first principles calculations. We predict a peculiar monoclinic phase (space group $P{2}_1/c$, 16 atoms/cell, $Z=4$ ) as the most stable phase for AlOOH above 340 GPa. The occurrence of this new phase results in the breakup of symmetric linear O-H-O hydrogen bonds into asymmetric, bent O-H-O linkages and in sevenfold coordinated metal cations. The new $P{2}_1/c$ phase turns out to be a universal high-pressure phase in group 13 oxide hydroxides, and stable for both compressed GaOOH and InOOH. The formation of the new phase in all compounds is favored by volume reduction due to denser packing.

Figure 1

Crystal structures of the (a) $P{2}_1/c$ phase of AlOOH, (b) δ-AlOOH, and (c) pyrite-type AlOOH. The blue, red, and black spheres represent aluminum, oxygen, and hydrogen atoms, respectively.

Figure 2

The calculated static enthalpies of various AlOOH structures. Left: comparing asymmetric and symmetric hydrogen bonding in both δ-AlOOH ( $\mathrm{CaC}{\mathrm{l}}_2$ structure type) (a) and $Pa$-3-AlOOH (pyrite structure type) (b). Right: comparing the high-pressure structures relative to δ-AlOOH, as a function of pressure; note the kink in the stability of $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+\mathrm{ice}$ due to a new $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ phase at 370 GPa.

Figure 3

Equations of state of the $\mathrm{CaC}{\mathrm{l}}_2$-, pyrite-, and monoclinic-type phases of AlOOH, as well as $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$. Solid and dashed lines indicate the symmetric and asymmetric hydrogen bonded structures, respectively. Vertical lines and arrows indicate transition pressures and are labeled with calculated volume collapses. The inserted picture shows the transitions from asymmetric to symmetric hydrogen bonding in both δ-AlOOH ( $\mathrm{CaC}{\mathrm{l}}_2$ structure type) and pyrite-type AlOOH at low pressure.

Figure 4

Phonon dispersions of $P{2}_1/c$ phase of AlOOH at 550 GPa. These phonon calculations demonstrated the structural stabilities of the predicted $P{2}_1/c$ phase of AlOOH in view of the absence of any imaginary phonons.

Figure 5

Finite temperature phase diagram of AlOOH at high pressures. Shown are stability boundaries of the pyrite phase (green line, taken from Ref. [45]) and the $P{2}_1/c$ phase (red line, our calculations). The black solid line indicates a potential temperature-pressure relation in a rocky super-Earth of 10 Earth masses [53, 54]. Gray (blue) dashed lines indicate the onset of ice melting (superionicity) from simulations [55, 56].

Figure 6

Calculated electronic band structure (left panel) and DOS (right panel) for the $P{2}_1/c$ structure of AlOOH at 400 GPa. The dashed black line is an indication of Fermi energy, which is set to zero.

Figure 7

The calculated static enthalpies of the structures relative to $Pa$-3 phase of GaOOH (a) and InOOH (b) structure as a function of pressure.

Figure 8

Phonon dispersions of $P{2}_1/c$ phase of InOOH at 70 GPa (a) and GaOOH at 200 GPa (b). These phonon calculations demonstrated the structural stabilities of the predicted $P{2}_1/c$ phase of InOOH and GaOOH in view of the absence of any imaginary phonons.

Figure 9

Calculated electronic band structure (left panel) and DOS (right panel) for the $P{2}_1/c$ structure of InOOH at 70 GPa (a) and GaOOH at 200 GPa (b). The dashed black line is an indication of Fermi energy, which is set to zero.

Figure 10

Equations of state of the $\mathrm{CaC}{\mathrm{l}}_2$-, pyrite-, and monoclinic-type phases of GaOOH (a) and InOOH (b), as well as $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3+\mathrm{ice}$ and $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3+\mathrm{ice}$. Solid and dashed lines indicate the symmetric and asymmetric hydrogen bonded structures, respectively. Vertical lines and arrows indicate transition pressures and are labeled with calculated volume collapse.