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Register a new userComputational searches for iron oxides at high pressures
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We have used density-functional-theory methods and the ab initio random structure searching (AIRSS) approach to predict stable structures and stoichiometries of mixtures of iron and oxygen at high pressures. Searching was performed for 12 different stoichiometries at pressures of 100, 350 and 500 GPa, which involved relaxing more than 32,000 structures. We find that Fe$_2$O$_3$ and FeO$_2$ are the only phases stable to decomposition at 100 GPa, while at 350 and 500 GPa several stoichiometries are found to be stable or very nearly stable. We report a new structure of Fe$_2$O$_3$ with $P2_12_12_1$ symmetry which is found to be more stable than the known Rh$_2$O$_3$(II) phase at pressures above $sim$233 GPa. We also report two new structures of FeO, with $Pnma$ and $Rbar{3}m$ symmetries, which are found to be stable within the ranges 195-285 GPa and 285-500 GPa, respectively, and two new structures of Fe$_3$O$_4$ with $Pca2_1$ and $P2_1/c$ symmetries, which are found to be stable within the ranges 100-340 GPa and 340-500 GPa, respectively. Finally, we report two new structures of Fe$_4$O$_5$ with $P4_2/n$ and $Pbar{3}m1$ symmetries, which are found to be stable within the ranges 100-231 GPa and 231-500 GPa, respectively. Our new structures of Fe$_3$O$_4$ and Fe$_4$O$_5$ are found to have lower enthalpies than their known structures within their respective stable pressure ranges.
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We investigate the binary phase diagram of helium and iron using first-principles calculations. We find that helium, which is a noble gas and inert at ambient conditions, forms stable crystalline compounds with iron at terapascal pressures. A FeHe compound becomes stable above 4 TPa, and a FeHe$_2$ compound above 12 TPa. Melting is investigated using molecular dynamics simulations, and a superionic phase with sublattice melting of the helium atoms is predicted. We discuss the implications of our predicted helium-iron phase diagram for interiors of giant (exo)planets and white dwarf stars.
Being a lithophile element at ambient pressure, magnesium is long believed to be immiscible with iron. A recent study by Gao et al. [1] showed that pressure turns magnesium into a siderophile element and can produce unconventional Fe-Mg compounds. Here, we extend the investigation to exoplanetary pressure conditions using an adaptive genetic algorithm-based variable-composition structural prediction approach. We identify several Fe-Mg phases up to 3 TPa. Our cluster alignment analysis reveals that most of the predicted Fe-Mg compounds prefer a BCC packing motif at terapascal pressures. This study provides a more comprehensive structure database to support future investigations of the high-pressure structural behavior of Fe-Mg and ternary, quaternary, etc. compounds involving these elements.
Recently, C. M. Pepin textit{et al.} [Science textbf{357}, 382 (2017)] reported the formation of several new iron polyhydrides FeH$_x$ at pressures in the megabar range, and spotted FeH$_5$, which forms above 130 GPa, as a potential high-tc superconductor, because of an alleged layer of dense metallic hydrogen. Shortly after, two studies by A.~Majumdar textit{et al.} [Phys. Rev. B textbf{96}, 201107 (2017)] and A.~G.~Kvashnin textit{et al.} [J. Phys. Chem. C textbf{122}, 4731 (2018)] based on {em ab initio} Migdal-Eliashberg theory seemed to independently confirm such a conjecture. We conversely find, on the same theoretical-numerical basis, that neither FeH$_5$ nor its precursor, FeH$_3$, shows any conventional superconductivity and explain why this is the case. We also show that superconductivity may be attained by transition-metal polyhydrides in the FeH$_3$ structure type by adding more electrons to partially fill one of the Fe--H hybrid bands (as, e.g., in NiH$_3$). Critical temperatures, however, will remain low because the $d$--metal bonding, and not the metallic hydrogen, dominates the behavior of electrons and phonons involved in the superconducting pairing in these compounds.
Hydrogen is the most abundant element in the universe, and its properties under conditions of high temperature and pressure are crucial to understand the interior of of large gaseous planets and other astrophysical bodies. At ultra high pressures solid hydrogen has been predicted to transform into a quantum fluid, because of its high zero point motion. Here we report first principles two phase coexistence and Z method determinations of the melting line of solid hydrogen in a pressure range spanning from 30 to 600 GPa. Our results suggest that the melting line of solid hydrogen, as derived from classical molecular dynamics simulations, reaches a minimum of 367 K at about 430 GPa, at higher pressures the melting line of the atomics Cs IV phase regain a positive slope. In view of the possible importance of quantum effects in hydrogen at such low temperatures, we also determined the melting temperature of the atomic CsIV phase at pressures of 400, 500, 600 GPa, employing Feynman path integral simulations. These result in a downward shift of the classical melting line by about 100 K, and hint at a possible secondary maximum in the melting line in the region between 500 and 600 GPa, testifying to the importance of quantum effects in this system. Combined, our results imply that the stability field of the zero temperature quantum liquid phase, if it exists at all, would only occur at higher pressures than previously thought.
By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H2 layers and the other layers of weakly coupled, disordered H2 molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm-1. At 275 GPa, our simulations predicted a transformation from po-hcp into the ordered molecular metallic Cmca phase (4 molecules/cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the po-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided theoretical support for claimed metallization of solid hydrogen below 300 GPa at 300 K.
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