Post-aragonite phases of CaCO$_{3}$ at lower mantle pressures

First principles linear response calculations are used to investigate the lattice dynamics of what is thought to be the third most abundant phase in the lower mantle, CaSiO_3 perovskite. The commonly assumed cubic structure (Pm3m) is found to be dynamically unstable at all pressures, exhibiting unstable modes along the Brillouin zone edge from the M-point to the R-point. Based on these results, we predict that the ground state structure of CaSiO_3 perovskite is a distorted ph...

We report the discovery of a new molecular phase of carbon dioxide at high-pressure and high-temperature. Using x-ray diffraction, we identify this phase as the theoretically predicted high-temperature Cmca phase [Bonev et al., Phys. Rev. Lett., 91, 065501 (2003)]. Its relation with phase III, on one hand, and its relative stability with respect to phase IV, on the other hand, are discussed based on spectroscopic and melting data. The existence of this strictly molecular phas...

There is a direct relation between the composition of a host star and that of the planets orbiting around it. As such, the recent discovery of stars with unusual chemical composition, notably enriched in carbon instead of oxygen, support the existence of exoplanets with a chemistry dominated by carbides instead of oxides. Accordingly several studies have been recently conducted on the Si C binary system at high pressure and temperature. Nonetheless, the properties of carbides...

We investigated the stability of polymeric CO2 over a wide range of pressures, temperatures, and chemical environments. We find that the I-42d polymeric structure, consisting of a three-dimensional network of corner sharing CO4 tetrahedra, forms at 40-140 GPa and from a CO-N2 mixture at 39 GPa. An exceptional stability field of 0 to 286 GPa and 100 to 2500 K is documented for this structure, making it a viable candidate for planetary interiors. The stability of the tetrahedra...

Carbon can exist as isolated dumbbell, one-dimensional (1D) chain, 2D plane, and 3D network in carbon solids or carbon-based compounds, which attributes to its rich chemical binding way, including sp-, sp2-, and sp3-hybridized bonds. Sp2 hybridizing carbon always captures special attention due to its unique physical and chemical property. Here, using evolutionary algorithm in conjunction with ab initio method, we found that under compression, dumbbell carbon in CaC2 can be po...

The distribution, recycling and storage of carbon in the Earth are of fundamental importance to understand the global carbon cycle between the deep Earth and near surface reservoirs. Degassing of CO2 at mid-ocean ridges may give information on the source region but the very low solubility of CO2 in tholeitic basalts has for consequence that near all Mid-Ocean Ridge Basalts glasses exsolve their CO2 rich vapor at shallow depth as they approach the ocean floor. Hence their CO2 ...

We report first-principles calculations of the frequency dependent linear and second-order optical properties of the two probable extended-solid phases of CO$_2$--V, i.e. $I\bar42d$ and $P2_12_12_1$. Compared to the parent $Cmca$ phase the linear optical susceptibility of both phases is much smaller. We find that $I\bar42d$ and $P2_12_12_1$ differ substantially in their linear optical response in the higher energy regime. The nonlinear optical responses of the two possible cr...

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...

The solid inner core of the Earth is predominantly composed of iron alloyed with several percent Ni and some lighter elements, Si, S, O, H, and C being the prime candidates. There have been a growing number of papers investigating C and H as possible light elements in the core, but the results are contradictory. Here, using ab initio simulations, we study the Fe-C and Fe-H systems at inner core pressures (330-364 GPa). Using the evolutionary structure prediction algorithm USP...

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc \rightarrow bcc \rightarrow simple cubic \rightarrow Ca-IV \rightarrow Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is simila...