465 research outputs found
Fe-C and Fe-H systems at pressures of the Earth's inner core
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 USPEX, we have determined the lowest-enthalpy structures
of possible carbides (FeC, Fe2C, Fe3C, Fe4C, FeC2, FeC3, FeC4 and Fe7C3) and
hydrides (Fe4H, Fe3H, Fe2H, FeH, FeH2, FeH3, FeH4) and have found that Fe2C
(Pnma) is the most stable iron carbide at pressures of the inner core, while
FeH, FeH3 and FeH4 are stable iron hydrides at these conditions. For Fe3C, the
cementite structure (Pnma) and the Cmcm structure recently found by random
sampling are less stable than the I-4 and C2/m structures found here. We found
that FeH3 and FeH4 adopt chemically interesting thermodynamically stable
structures, in both compounds containing trivalent iron. The density of the
inner core can be matched with a reasonable concentration of carbon, 11-15
mol.percent (2.6-3.7 wt.percent) at relevant pressures and temperatures. This
concentration matches that in CI carbonaceous chondrites and corresponds to the
average atomic mass in the range 49.3-51.0, in close agreement with inferences
from the Birch's law for the inner core. Similarly made estimates for the
maximum hydrogen content are unrealistically high, 17-22 mol.percent (0.4-0.5
wt.percent), which corresponds to the average atomic mass in the range
43.8-46.5. We conclude that carbon is a better candidate light alloying element
than hydrogen.Comment: Published in Physics-Uspekhi: full text will soon appear at
http://ufn.ru/en/articles/2012/5/c/ (currently, only abstract is available
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
The elastic constants of MgSiO3 perovskite at pressures and temperatures of the Earth's mantle
The temperature anomalies in the Earth's mantle associated with thermal
convection1 can be inferred from seismic tomography, provided that the elastic
properties of mantle minerals are known as a function of temperature at mantle
pressures. At present, however, such information is difficult to obtain
directly through laboratory experiments. We have therefore taken advantage of
recent advances in computer technology, and have performed finite-temperature
ab initio molecular dynamics simulations of the elastic properties of MgSiO3
perovskite, the major mineral of the lower mantle, at relevant thermodynamic
conditions. When combined with the results from tomographic images of the
mantle, our results indicate that the lower mantle is either significantly
anelastic or compositionally heterogeneous on large scales. We found the
temperature contrast between the coldest and hottest regions of the mantle, at
a given depth, to be about 800K at 1000 km, 1500K at 2000 km, and possibly over
2000K at the core-mantle boundary.Comment: Published in: Nature 411, 934-937 (2001
The high-pressure phase of boron, {\gamma}-B28: disputes and conclusions of 5 years after discovery
{\gamma}-B28 is a recently established high-pressure phase of boron. Its
structure consists of icosahedral B12 clusters and B2 dumbbells in a NaCl-type
arrangement (B2){\delta}+(B12){\delta}- and displays a significant charge
transfer {\delta}~0.5- 0.6. The discovery of this phase proved essential for
the understanding and construction of the phase diagram of boron. {\gamma}-B28
was first experimentally obtained as a pure boron allotrope in early 2004 and
its structure was discovered in 2006. This paper reviews recent results and in
particular deals with the contentious issues related to the equation of state,
hardness, putative isostructural phase transformation at ~40 GPa, and debates
on the nature of chemical bonding in this phase. Our analysis confirms that (a)
calculations based on density functional theory give an accurate description of
its equation of state, (b) the reported isostructural phase transformation in
{\gamma}-B28 is an artifact rather than a fact, (c) the best estimate of
hardness of this phase is 50 GPa, (d) chemical bonding in this phase has a
significant degree of ionicity. Apart from presenting an overview of previous
results within a consistent view grounded in experiment, thermodynamics and
quantum mechanics, we present new results on Bader charges in {\gamma}-B28
using different levels of quantum-mechanical theory (GGA, exact exchange, and
HSE06 hybrid functional), and show that the earlier conclusion about
significant degree of partial ionicity in this phase is very robust
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