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

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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“…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: 87%

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.

“…These are the same structures that were found as high-pressure candidates in potassium hydroxide and that formed the basis of a successful refinement of neutron powder diffraction data of KOH-VI. 20 The NaOH-V phase, suggested in EDX experiments, is not energetically competitive at high pressures, and neither is any of the other known alkali hydroxide structure types.…”

Section: High-pressure Phase Evolution In Rbohmentioning

confidence: 97%

“…15 Recently, first-principles calculations together with neutron powder diffraction were able to identify the KOH-VI structure as having an unusual hydrogen-bond network topology, with localised (OH) 4 units in a matrix of potassium cations. 20 Likewise, the high-pressure phase LiOH-III, stable above 0.7 GPa, was shown in calculations (analysing neutron diffraction patterns, vibrational and energetic properties) to be a new structure type with linear OHÁ Á ÁOH chains. Calculations also predicted a high-pressure phase LiOH-IV (above 17 GPa) that is structurally identical to NaOH-V. 21 Here, the thread of theoretical studies of the compressed alkali hydroxides is continued, and the high-pressure phase transitions exhibited by RbOH and CsOH are examined.…”

Section: Introductionmentioning

confidence: 97%

High-pressure phase transitions in rubidium and caesium hydroxides

Phys. Chem. Chem. Phys.

2016

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A computational investigation of the high-pressure phase sequence of the heaviest alkali hydroxides, RbOH and CsOH, shows that the phase diagram of both compounds is richer than hitherto thought. First-principles calculations suggest, based on energetics and comparisons to experimental diffraction and spectroscopy signatures, that the high-pressure phase RbOH-VI, stable above 6 GPa in experiment, should be assigned the KOH-VI structure type, and features localised hydrogen-bonded (OH)4 units. Meanwhile, a new high-pressure phase CsOH-VII is predicted to be stable above 10 GPa in an isosymmetric phase transition that, like RbOH-VI, marks the transition from layered to three-dimensional network structures under increased compression. Both new phases highlight an unexpected flexibility of hydrogen bond network formation in a series of compounds that seemingly only vary in the cation size, and potential consequences for similar systems, such as water-carrying minerals, are discussed briefly.