192 research outputs found
A many-body interatomic potential for ionic systems: application to MgO
An analytic representation of the short-range repulsion energy in ionic
systems is described that allows for the fact that ions may change their size
and shape depending on their environment. This function is extremely efficient
to evaluate relative to previous methods of modeling the same physical effects.
Using a well-defined parametrization procedure we have obtained parameter sets
for this energy function that reproduce closely the density functional theory
potential energy surface of bulk MgO. We show how excellent agreement can be
obtained with experimental measurements of phonon frequencies and temperature
and pressure dependences of the density by using this effective potential in
conjunction with ab initio parametrization.Comment: To appear in Journal of Chemical Physics (Oct 15th 2003
First-principles thermoelasticity of bcc iron under pressure
We investigate the elastic and isotropic aggregate properties of
ferromagnetic bcc iron as a function of temperature and pressure by computing
the Helmholtz free energies for the volume-conserving strained structures using
the first-principles linear response linear-muffin-tin-orbital method and the
generalized-gradient approximation. We include the electronic excitation
contributions to the free energy from the band structures, and phonon
contributions from quasi-harmonic lattice dynamics. We make detailed
comparisons between our calculated elastic moduli and their temperature and
pressure dependences with available experimental and theoretical data.Comment: 5 figures, 2 table
Structures and stability of calcium and magnesium carbonates at mantle pressures
Ab initio random structure searching (AIRSS) and density functional theory
methods are used to predict structures of calcium and magnesium carbonate
(CaCO and MgCO) at high pressures. We find a previously unknown
CaCO structure which is more stable than the aragonite and "post aragonite"
phases in the range 32--48 GPa. At pressures from 67 GPa to well over 100 GPa
the most stable phase is a previously unknown CaCO structure of the
pyroxene type with fourfold coordinated carbon atoms. We also predict a stable
structure of MgCO in the range 85--101 GPa. Our results lead to a revision
of the phase diagram of CaCO over more than half the pressure range
encountered within the Earth's mantle, and smaller changes to the phase diagram
of MgCO. We predict CaCO to be more stable than MgCO in the Earth's
mantle above 100 GPa, and that CO is not a thermodynamically stable
compound under deep mantle conditions. Our results have significant
implications for understanding the Earth's deep carbon cycle.We acknowledge financial support from the Engineering and Physical Sciences Research Council United Kingdom (EPSRC) of the United Kingdom.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.91.10410
High-Temperature Thermodynamic Properties Of Forsterite
The high-temperature thermodynamic properties of forsterite were reviewed in the light of a new determination of the isobaric heat capacity (C(p)), up to 1850 K, and Raman spectroscopic measurements, up to 1150 K and 10 GPa. The C(p) measurements and available data on thermal expansion (alpha) and bulk modulus (K) show that the isochoric specific heat (C-nu) exceeds the harmonic limit of Dulong and Petit above 1300 K. This intrinsic anharmonic behavior of C-nu) can be modeled by introducing anharmonic parameters a(i) = (partial-lnv(i)/partial-T)V which are calculated from the measured pressure and temperature shifts of the vibrational frequencies. These parameters are all negative, with absolute values lower for the stretching modes of the SiO4 tetrahedra (a(i) almost-equal-to - 1 x 10(-5) K-1) than for the lattice modes (a(i) almost-equal-to 2 x 10(-5) K-1). Through the relation C(p) = C-nu) + alpha-2K(T)VT, the calculated anharmonic C-nu) and the measured C(p) are then used to determine the temperature dependences of the thermal expansion and bulk modulus of forsterite, up to 2000 K, in agreement with recent experimental results. Finally, all these data point to an inconsistency for the Gruneisen parameter of forsterite, whereby the macroscopic parameter gamma = alpha-VK(T)/C-nu) cannot be evaluated simply at high temperature by summation of the individual isothermal mode Gruneisen parameters gamma-iT = K(T) (partial-lnv(i)/partial-P)
quasiharmonic equations of state for dynamically-stabilized soft-mode materials
We introduce a method for treating soft modes within the analytical framework
of the quasiharmonic equation of state. The corresponding double-well
energy-displacement relation is fitted to a functional form that is harmonic in
both the low- and high-energy limits. Using density-functional calculations and
statistical physics, we apply the quasiharmonic methodology to solid periclase.
We predict the existence of a B1--B2 phase transition at high pressures and
temperatures
Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D" layer
The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO3
perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3). But it has not been
possible to explain many unusual properties of the lowermost 150 km of the
mantle (the D" layer) with this mineralogy. Here, using ab initio simulations
and high-pressure experiments, we show that at pressures and temperatures of
the D" layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type
post-perovskite phase. The elastic properties of the post-perovskite phase and
its stability field explain several observed puzzling properties of the D"
layer: its seismic anisotropy, the strongly undulating shear-wave discontinuity
at its top and possibly the anticorrelation between shear and bulk sound
velocities.Comment: PUBLISHED IN Nature 430, 445-448 (2004
Experimental investigation of the stability of Fe-rich carbonates in the lower mantle
International audienceThe fate of carbonates in the Earth's mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earth's lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earth's lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth
Thermodynamically Consistent Force Field for Molecular Dynamics Simulations of Alkaline-Earth Carbonates and Their Aqueous Speciation
In recent years atomistic simulations have become increasingly important in providing molecular insight to complement experiments. Even for the seemingly simple case of ion-pair formation a detailed atomistic picture of the structure and relative stability of the contact, solvent-shared and solvent-separated ion pairs can only be readily achieved by computer simulation. Here a new force field parametrization for the alkaline-earth carbonate interactions in water has been developed by fitting against experimental thermodynamic and structural data. We demonstrate that the present force field can accurately reproduce the dynamics and thermodynamics of the ions in solution, which is the key to producing quantitatively accurate data that can be compared against experiment
Composition of the Earth's Inner Core from High-pressure Sound Velocity Measurements in Fe-Ni-Si alloys
International audienceWe performed room-temperature sound velocity and density measurements on a polycrystalline alloy, Fe0.89Ni0.04Si0.07, in the hexagonal close-packed (hcp) phase up to 108 GPa. Over the investigated pressure range the aggregate compressional sound velocity is âŒ9% higher than in pure iron at the same density. The measured aggregate compressional (VP) and shear (VS) sound velocities, extrapolated to core densities and corrected for anharmonic temperature effects, are compared with seismic profiles. Our results provide constraints on the silicon abundance in the core, suggesting a model that simultaneously matches the primary seismic observables, density, P-wave and S-wave velocities, for an inner core containing 4 to 5 wt.% of Ni and 1 to 2 wt.% of Si
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