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Register a new userUniversal conductivity and the electrodynamics of graphite at high pressures
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We address the in-plane pressure-dependent electrodynamics of graphite through synchrotron based infrared spectroscopy and ab initio Density Functional Theory calculations. The Drude term remarkably increases upon pressure application, as a consequence of an enhancement of both electron and hole charge densities. This is due to the growth of the band dispersion along the k_z direction between the K and H points of the Brillouin zone. On the other hand, the mid-infrared optical conductivity between 800 and 5000 cm-1 is almost flat, and very weakly pressure dependent, at least up to 7 GPa. This demonstrates a surprising robustness of the graphene-like universal quantum conductance of graphite, even when the interlayer distance is significantly reduced.
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We have performed a combined experimental and theoretical study of ethane and methane at high pressures up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopy and the USPEX ab-initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (Phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on a good agreement of the experimental results and theoretical predictions. For methane, our XRD measurements are in agreement with the previously reported high-pressure structures and EOS. We have determined the equations of state of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.
Exposure of highly oriented pyrolytic graphite to bromine vapor gives rise to in-plane charge conductivities which increase monotonically with intercalation time toward values (for ~6 at% Br) that are significantly higher than Cu at temperatures down to 5 K. Magnetotransport, optical reflectivity and magnetic susceptibility measurements confirm that the Br dopes the graphene sheets with holes while simultaneously increasing the interplanar separation. The increase of mobility (~ 5E4 cm^2/Vs at T=300 K) and resistance anisotropy together with the reduced diamagnetic susceptibility of the intercalated samples suggests that the observed supermetallic conductivity derives from a parallel combination of weakly-coupled hole-doped graphene sheets.
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.
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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