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Register a new userBond strengthening in dense H2O and implications to planetary composition
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H2O is an important constituent in planetary bodies, controlling habitability and, in geologically-active bodies, plate tectonics. At pressures within the interior of many planets, the H-bonds in H2O collapse into stronger, ionic bonds. Here we present agreement between X-ray diffraction and Raman spectroscopy for the transition from ice-VII to ice-X occurring at a pressure of approximately 30.9 GPa by means of combining grain normalizing heat treatment via direct laser heating with static compression. This is evidenced by the emergence of the characteristic Raman mode of cuprite-like ice-X and an abrupt 2.5-fold increase in bulk modulus, implying a significant increase in bond strength. This is preceded by a transition from cubic ice-VII to a structure of tetragonal symmetry, ice-VIIt at 5.1 GPa. Our results significantly shift the mass/radius relationship of water-rich planets and define a high-pressure limit for release of chemically-bound water within the Earth, making the deep mantle a potential long-term reservoir of ancient water.
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Liquid metallic hydrogen (LMH) was recently produced under static compression and high temperatures in bench-top experiments. Here, we report a study of the optical reflectance of LMH in the pressure region of 1.4-1.7 Mbar and use the Drude free-electron model to determine its optical conductivity. We find static electrical conductivity of metallic hydrogen to be 11,000-15,000 S/cm. A substantial dissociation fraction is required to best fit the energy dependence of the observed reflectance. LMH at our experimental conditions is largely atomic and degenerate, not primarily molecular. We determine a plasma frequency and the optical conductivity. Properties are used to analyze planetary structure of hydrogen rich planets such as Jupiter.
Dislocations govern the properties of any crystals. Yet, how dislocation of pentagonheptagon (5|7) in grain boundaries (GBs) affects the mechanical properties of two-dimensional MoS2 crystals remains poorly known. Using atomistic simulations and continuum disclination dipole model, we show that, depending on the tilt angle and 5|7 dislocation arrangement, MoS2 GB strength can be enhanced or reduced with tilt angle. For zigzag-tilt GBs primarily composed of Mo5|7+S5|7 dislocations, GB strength monotonically increases as the square of tilt angle. For armchair-tilt GBs with Mo5|7 or S5|7 dislocations, however, the trend of GB strength breaks down as 5|7 dislocations are non-evenly spaced. Moreover, mechanical failure initiates at the bond shared by 5|7 rings, in contrast to graphene where failure occurs at the bond shared by 6|7 rings. This work provides new insights into mechanical design of synthetic transition metal dichalcogenide crystals via dislocation engineering.
The scaling of the bond-bond correlation function $C(s)$ along linear polymer chains is investigated with respect to the curvilinear distance, $s$, along the flexible chain and the monomer density, $rho$, via Monte Carlo and molecular dynamics simulations. % Surprisingly, the correlations in dense three dimensional solutions are found to decay with a power law $C(s) sim s^{-omega}$ with $omega=3/2$ and the exponential behavior commonly assumed is clearly ruled out for long chains. % In semidilute solutions, the density dependent scaling of $C(s) approx g^{-omega_0} (s/g)^{-omega}$ with $omega_0=2-2 u=0.824$ ($ u=0.588$ being Florys exponent) is set by the number of monomers $g(rho)$ contained in an excluded volume blob of size $xi$. % Our computational findings compare well with simple scaling arguments and perturbation calculation. The power-law behavior is due to self-interactions of chains on distances $s gg g$ caused by the connectivity of chains and the incompressibility of the melt. %
The moon-forming impact and the subsequent evolution of the proto-Earth is strongly dependent on the properties of materials at the extreme conditions generated by this violent collision. We examine the high pressure behavior of MgO, one of the dominant constituents in the earths mantle, using high-precision, plate impact shock compression experiments performed on Sandia National Laboratories Z-Machine and extensive quantum simulations using Density Functional Theory (DFT) and quantum Monte Carlo (QMC). The combined data span from ambient conditions to 1.2 TPa and 42,000 K, showing solid-solid and solid-liquid phase boundaries. Furthermore our results indicate under impact that the solid and liquid phases coexist for more than 100 GPa, pushing complete melting to pressures in excess of 600 GPa. The high pressure required for complete shock melting places a lower bound on the relative velocities required for the moon forming impact.
Using density functional theory implemented within the generalized gradient approximation, a new non-magnetic insulating ground state of solid oxygen is proposed and found to be energetically favored at pressures corresponding to the $epsilon$-phase. The newly-predicted ground state is composed of linear herringbone-type chains of O$_2$ molecules and has {it Cmcm} symmetry (with an alternative monoclinic cell). Importantly, this phase supports IR-active zone-center phonons, and their computed frequencies are found to be in broad agreement with recent infrared absorption experiments.
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