Lithium hydroxide, LiOH, at elevated densities

Proc. Natl. Acad. Sci. U.S.A.

Mookherjee

2016

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We investigate the high-pressure phase diagram of the hydrous mineral brucite, Mg(OH) 2 , using structure search algorithms and ab initio simulations. We predict a high-pressure phase stable at pressure and temperature conditions found in cold subducting slabs in Earth's mantle transition zone and lower mantle. This prediction implies that brucite can play a much more important role in water transport and storage in Earth's interior than hitherto thought. The predicted high-pressure phase, stable in calculations between 20 and 35 GPa and up to 800 K, features MgO 6 octahedral units arranged in the anatase-TiO 2 structure. Our findings suggest that brucite will transform from a layered to a compact 3D network structure before eventual decomposition into periclase and ice. We show that the high-pressure phase has unique spectroscopic fingerprints that should allow for straightforward detection in experiments. The phase also has distinct elastic properties that might make its direct detection in the deep Earth possible with geophysical methods.brucite | pressure | phase transition | electronic structure calculations W ater plays an important role in sustaining geological activity. For instance, water helps in lowering the mantle's melting temperature, enhancing diffusion and creep thus affecting rheology of rocks, and also influences mineral phase boundaries. Current estimates suggest that the Earth's mantle is likely to contain a mass of water equivalent to the mass of the world's oceans (1, 2). The exchange of water between the surface and deep mantle reservoirs is vital for the sustenance of surface water over geological time scales (3). Hydrous minerals stable in the hydrated oceanic crust and mantle play an important role in transporting water into the Earth's interior. Hydrated peridotite, the major mantle rock type, can be understood by considering mineral phases stable in the ternary system of MgO-SiO 2 -H 2 O (MSH). Brucite, Mg(OH) 2 , is arguably the simplest hydrous mineral in the MSH system. Brucite is also the most important MgO-H 2 O binary and the most water-rich phase within the MSH ternary system.The crystal structure of brucite consists of Mg 2+ cations and OH − anions arranged in layers, in an overall trigonal structure (space group symmetry P 3m1); Fig. 1. The common ionic compound CdI 2 is the archetype crystal structure for brucite as well as for portlandite [Ca(OH) 2 )] and several other transition metal hydroxides M(OH) 2 where M = Mn, Ni, Co, Fe, Cd,. In brucite, the crystal structure comprises layers of edge-sharing MgO 6 polyhedra. The interaction between the layers is weak at ambient conditions, where each upward-pointing OH group is surrounded by three downward-pointing OH groups in the adjacent layer and vice versa. Under compression the H-H repulsive interactions lead to positional disordering of the protons, which are displaced from the 2d Wyckoff site into one of three equivalent 6i sites as documented from neutron diffraction studies (11,12). Vibrational spectroscopic studies incl...

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confidence: 99%

High-pressure phase of brucite stable at Earth’s mantle transition zone and lower mantle conditions

Proc. Natl. Acad. Sci. U.S.A.

Mookherjee

2016

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“…As seen in other molecular crystals with strong partial charge transfers, we find in our calculations that the band gap in potassium hydroxide increases with increasing pressure. 12,57,58 Note that the band gaps obtained from semilocal exchange-correlation functionals are underestimated due to their nondiscontinuity of the exchange-correlation potential 59 but should describe trends of gap opening or closure correctly. More accurate quasiparticle energies and gaps can be obtained by using the GW method, and optical gaps by solving the Bethe-Salpeter equation for correlated electron-hole pairs.…”

Section: Discussionmentioning

confidence: 99%

“…The nature of those hydrogen bond networks, and their manipulation by changes in temperature or external pressure, has been the subject of numerous experimental and computational studies. [1][2][3][4][5][6][7][8][9][10][11][12] The response of the hydrogen bond network structure to increasing density in these systems is of particular interest. For instance, there is considerable evidence that the OH − anion behaves differently from molecular OH groups, but the origin of this phenomenon is not fully resolved.…”

Section: Introductionmentioning

confidence: 99%

Pressure-induced localisation of the hydrogen-bond network in KOH-VI

The Journal of Chemical Physics

Guthrie

Nelmes

et al. 2015

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Using a combination of ab initio crystal structure prediction and neutron diffraction techniques, we have solved the full structure of KOH-VI at 7 GPa. Rather than being orthorhombic and proton-ordered as had previously be proposed, we find that this high-pressure phase of potassium hydroxide is tetragonal (space group I4/mmm) and proton disordered. It has an unusual hydrogen bond topology, where the hydroxyl groups form isolated hydrogen-bonded square planar (OH)4 units. This structure is stable above 6.5 GPa and, despite being macroscopically proton-disordered, local ice rules enforce microscopic order of the hydrogen bonds. We suggest the use of this novel type of structure to study concerted proton tunneling in the solid state, while the topology of the hydrogen bond network could conceivably be exploited in data storage applications based solely on the manipulations of hydrogen bonds. The unusual localisation of the hydrogen bond network under applied pressure is found to be favored by a more compact packing of the constituents in a distorted cesium chloride structure.

“…A c c e p t e d M a n u s c r i p t transitions and to corroborate (or even re-interpret) experimental structure solutions [128][129][130] . For Mg(OH) 2 , a similar transformation is predicted by CSP to take place (CALYPSO has confirmed these results) [123] : the layered brucite structure becomes unstable under pressure and is superseded by a three-dimensional network of corner-sharing polyhedra that is topologically equivalent to TiO 2 anatase (see insets in Figure 10), with OH groups forming very short hydrogen bonds in channels of the heavy atom network.…”

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confidence: 85%

Geoscience material structures prediction via CALYPSO methodology

2019

Chinese Phys. B

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Many properties of planets such as their interior structure and thermal evolution depend on the highpressure properties of their constituent materials. This paper reviews how crystal structure prediction methodology can help shed light on the transformations materials undergo at the extreme conditions inside planets. The discussion focuses on three areas: (i) the propensity of iron to form compounds with volatile elements at planetary core conditions (important to understand the chemical makeup of Earth's inner core), (ii) the chemistry of mixtures of planetary ices (relevant for the mantle regions of giant icy planets), and (iii) examples of mantle minerals. In all cases the abilities and current limitations of crystal structure prediction are discussed across a range of example studies.