Structures and stability of calcium and magnesium carbonates at mantle pressures

Crystal structure prediction methods and first-principles calculations have been used to explore low-energy structures of carbon monoxide (CO). Contrary to the standard wisdom, the most stable structure of CO at ambient pressure was found to be a polymeric structure of Pna21 symmetry rather than a molecular solid. This phase is formed from six-membered (4 Carbon + 2 Oxygen) rings connected by C=C double bonds with two double-bonded oxygen atoms attached to each ring. Interest...

We study the formation of calcium carbonate, through the solid-gas interaction of amorphous Ca-silicate with gaseous CO2, at elevated pressures, and link this to the possible presence of calcium carbonate in a number of circumstellar and planetary environments. We use in-situ synchrotron X-Ray powder diffraction to obtain detailed structural data pertaining to the formation of the crystalline calcium carbonate phase vaterite and its evolution with temperature. We found that t...

Crystal structure prediction with theoretical methods is particularly challenging when unit cells with many atoms need to be considered. Here we employ a symmetry-driven structure search (SYDSS) method and combine it with density functional theory (DFT) to predict novel crystal structures at high pressure. We sample randomly from all 1,506 Wyckoff positions of the 230 space groups to generate a set of initial structures. During the subsequent structural relaxation with DFT, e...

Understanding the high-pressure behavior of C-H system is of great importance due to its key role in organic, bio-, petroleum and planetary chemistry. We have performed a systematic investigation of the pressure-composition phase diagram of the C-H system at pressures up to 400 GPa using evolutionary structure prediction coupled with ab initio calculations and discovered that only saturated hydrocarbons are thermodynamically stable. Several stable methane-hydrogen co crystals...

Simple cubic (SC) phase has been long experimentally determined as the high-pressure phase III of elemental calcium (Ca) since 1984. However, recent density functional calculations within semi-local approximation showed that this SC phase is structurally unstable by exhibiting severely imaginary phonons, and is energetically unstable with respect to a theoretical body-centered tetragonal I41/amd structure over the pressure range of phase III. These calculations generated exte...

Alkaline-earth metal oxides, in particular MgO and CaO dominate Earths lower mantle, therefore, exploring high pressure behavior of this class of compounds is of significant geophysical research interest. Among all these compounds, BaO exhibits rich polymorphism in the pressure range of 0-1.5 Mbar. Static enthalpy calculations revealed that BaO undergoes a pressure induced structural phase transition from NaCl-type (B1) $\rightarrow$ NiAs-type (B8) $\rightarrow$ distorted CsC...

The Earth's crust is known to be depleted of gold, among other slightly heavy noble metals transported by magma from the Earth's mantle to the crust. The bulk silicate Earth (BSE) model also suggests significant depletion of Au in the silicate mantle itself, which cannot be explained by the amount of Au in the mantle's magma. This implies that Au could remain in the lower mantle and form stable compounds, especially with iron, which is the predominant element within the core....

On the basis of ab-initio pseudopotential calculations, we study structural, magnetic, dynamical, and mechanical properties of the hypothetical CaC ionic compound in the rock-salt (RS), B2, zinc-blende (ZB), wurtzite (WZ), NiAs (NA), anti-NiAs (NA*), and CrB (B33) structures. It is argued that the ZB, WZ, NA, and RS structures are more ionic while the NA*, B2, and B33 structures are more covalent systems. As a result of that, the nonmagnetic B33-CaC is the energetically prefe...

Earths lower mantle extending from 670 to 2,990 km deep is predominantly composed of a perovskite-type (Mg,Fe)SiO3 phase1,2. The perovskite phase undergoes a structural phase transition to a post-perovskite phase responsible for D" layer seismic discontinuity2,3 at about 2690 km depth in the lowermost region of the lower mantle. However, structural basis of other seismic discontinuities occurring in the upper region of the lower mantle (700 km to 1,200 km deep) remains unexpl...

Silica is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. As the simplest silicates, understanding pure silica is a prerequisite to understanding the rocky part of the Earth, its majority. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates. Here, we study silica with quantum Monte Carlo (QMC), which until now was not computationally possible fo...