27 results on '"Vocadlo, L"'
Search Results
2. High pressure stability of the monosilicides of cobalt and the platinum group elements
- Author
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Hernandez, J.A., Vočadlo, L., and Wood, I.G.
- Published
- 2015
- Full Text
- View/download PDF
Catalog
3. Phonon Density of States of Iron up to 153 Gigapascals
- Author
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Mao, H. K., Xu, J., Struzhkin, V. V., Shu, J., Hemley, R. J., Sturhahn, W., Hu, M. Y., Alp, E. E., Vocadlo, L., Alfè, D., Price, G. D., Gillan, M. J., Schwoerer-Böhning, M., Häusermann, D., Eng, P., Shen, G., Giefers, H., Lübbers, R., and Wortmann, G. more...
- Published
- 2001
4. Single-crystal analysis of disorder in deuterated and protonated mirabilite
- Author
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Wood, I. G., Brand, H. E. A., Fortes, A. D., Vocadlo, L., and Gutmann, M.
- Published
- 2010
5. Probing the high pressure stability of ammonia dihydrate phase IV
- Author
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Fortes, A. D., Wood, I. G., Vocadlo, L., Brand, H. E. A., Grindrod, P. M., Joy, K. H., and Tucker, M. G.
- Published
- 2008
6. Crystal structures of methanol-water and methanol-ammonia compounds
- Author
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Fortes, A. D., Wood, I. G., Brand, H. E. A., Vocadlo, L., and Knight, K. S.
- Published
- 2007
7. The thermal expansivity of mirabilite (Na₂SO₄·10D₂O)
- Author
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Fortes, A. D., Wood, I. G., Brand, H. E. A., Vocadlo, L., and Knight, K. S.
- Published
- 2006
8. The phase behaviour of epsomite to 50 kbar
- Author
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Fortes, A. D., Wood, I. G., Vocadlo, L., Brand, H. E. A., Grindrod, P. M., Joy, K. H., and Tucker, M. G.
- Published
- 2005
9. The thermal expansion and phase transitions of anhydrous MgSO₄
- Author
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Fortes, A. D., Wood, I. G., Vocadlo, L., Brand, H. E. A., and Knight, K. S.
- Published
- 2005
10. The equation of state of ammonia hemihydrate to 4.5 kbar
- Author
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Fortes, A. D., Wood, I. G., Vocadlo, L., Brand, H. E. A., and Knight, K. S.
- Published
- 2005
11. Gruneisen parameters and isothermal equations of state.
- Author
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Vocadlo, L. and Poirer, J.P.
- Subjects
- *
EQUATIONS of state , *PRESSURE , *LOGARITHMIC functions - Abstract
Focuses on a study that assessed some of the common definitions of the Gruneisen parameter quantity in geophysics and the isothermal equations of state. Process of fitting equations of state to ab initio pressure-volume data; Determination of Gruneisen parameter using the fourth order of logarithmic equation of state; Discussion and conclusion. more...
- Published
- 2000
12. The structure, composition and evolution of Mercury's core
- Author
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Edgington, A. L., Vocadlo, L., Stixrude, L., and Wood, I. G.
- Subjects
550 - Abstract
This thesis presents the results of ab initio molecular dynamics calculations of the adiabatic gradient of pure liquid iron and a liquid Fe-S-Si mixture with the relative atomic percentages, 80:10:10. Laser-heated diamond-anvil-cell experiments have been conducted to measure the liquidus and solidus relationships of Fe0.8S0.1Si0.1. First-principles molecular dynamics is combined with thermodynamic integration and free-energy minimisation to simulate the spin transition in pure liquid iron and liquid Fe0.8S0.1Si0.1. From the magnetic transition, the equations of state, thermodynamic properties and adiabatic gradients of pure liquid iron and liquid Fe0.8S0.1Si0.1 are determined. The calculated adiabatic gradients are used alongside the gradients of published melting curves of iron and the measured liquidus of Fe0.8S0.1Si0.1 to provide insight into the crystallisation regime of the core of Mercury. The suggested crystallisation regime of a hypothetical pure iron or Fe0.8S0.1Si0.1 Mercurian core depends strongly on the derivative of the melting curve. Results suggest that a Fe0.8S0.1Si0.1 core of Mercury may start in a ‘top-down’ crystallisation regime resulting in a complex core structure with a possible stratified Fe-S layer at the top of the core, which, may in-part explain the observed weak magnetic field of Mercury. more...
- Published
- 2016
13. The thermoelastic properties of post-perovskite analogue phases
- Author
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Lindsay-Scott, A., Brodholt, J., Dobson, D., Vocadlo, L., and Wood, I. G.
- Subjects
550 - Abstract
Post-perovskite MgSiO3 is a major component of the D'' zone at the base of the lower mantle, so knowledge of its physical properties is essential to understanding mantle dynamics. Unfortunately, MgSiO3 post-perovskite is stable only at Mbar pressures. This difficulty can be addressed by combining computer simulations with experiments on analogue post-perovskite ABX3 phases, stable at ambient pressure and temperature. In this project, the properties of MgSiO3 and other oxide and fluoride post-perovskites were simulated using both athermal and molecular dynamics ab initio methods. Correspondence with experiment was generally good, although better in compression than when heated. Because few ABO3 oxides with this structure are known, an empirical approach was developed to predict whether post-perovskite phases would occur for given A, B and X elements, and to estimate the stabilisation pressures required. Only CaIrO3 forms at ambient pressure; single crystals were prepared for structural measurements, but the extremely high absorption made X-ray diffraction unreliable. Powdered CaPtO3 was synthesised at high-pressure and its thermoelastic properties and structure determined at high-pressure and from low to high-temperature by neutron powder diffraction; its isothermal equation of state was determined by Xray powder diffraction. CaPtO3 (and CaIrO3 as measured previously) have the same axial compression sequence (k_^c > k_^a > k_^b) as MgSiO3, but all show different axial expansion at high-temperature. Across D'' significant changes in the physical properties of MgSiO3 post-perovskite are likely to arise from changes in temperature as well as from changes in pressure; the differences in axial expansion therefore suggest that CaPtO3 and CaIrO3 may not be suitable analogues. Computer simulations suggest that ABF3 compounds may provide better analogues for MgSiO3 postperovskite. A new post-perovskite NaNiF3 has been synthesised at 15 GPa and its unit cell parameters determined; further experiments on these fluorides are the subject of ongoing work. more...
- Published
- 2011
14. Computational and experimental studies of solids in the ammonia-water system
- Author
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Fortes, Andrew, Vocadlo, L., Brodholt, J. P., and Wood, I. G.
- Subjects
546.71120285 - Abstract
This thesis reports the results of first-principles computational studies of thirteen crystalline structures in the H2O-NH3 system. This includes eight low- and highpressure polymorphs of pure water ice, two polymorphs of solid ammonia, and three low-pressure stoichiometric ammonia hydrates. These simulations have been used to determine the athermal equation of state (EoS) of each phase. Where empirical data was lacking, experiments have been undertaken. Hence, this thesis also reports the results of time-of-flight neutron scattering studies of deuterated ammonia dihydrate powders down to 4 K, and up to a maximum pressure of 8.6 GPa. In addition, I have developed a flexible and accurate planetary model that can be used to calculate the triaxial shape and gravitational field of any object, regardless of size or composition, given an assumed mineralogical constitution and provided the EoS of said minerals are known. The EoS parameters found in this work have therefore been used to model the structure and thermal evolution of icy moons orbiting Saturn in anticipation of the Cassini spacecraft arriving at Saturn in mid-2004. Models of Rhea, Saturn’s second largest moon, suggest that its volatile component is likely to contain > 3 weight percent ammonia, but that one is unlikely to be able to constrain the bulk chemistry of the ice mantle from Cassini flyby data. more...
- Published
- 2004
15. Carbon Partitioning Between the Earth's Inner and Outer Core
- Author
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Dario Alfè, Yunguo Li, Lidunka Vočadlo, John P. Brodholt, Li, Y., Vocadlo, L., Alfe, D., and Brodholt, J.
- Subjects
thermodynamic integration ,defect ,inner-core boundary ,Materials science ,molecular dynamic ,chemistry.chemical_element ,Thermodynamic integration ,Outer core ,thermodynamics ,Molecular dynamics ,Geophysics ,chemistry ,Space and Planetary Science ,Geochemistry and Petrology ,Chemical physics ,partitioning ,Earth and Planetary Sciences (miscellaneous) ,Carbon ,Earth (classical element) - Abstract
Knowledge of the abundance and distribution of light elements in the core is fundamental to the understanding of the Earth and other planetary systems. Recent studies (Li et al., 2018; Mashino et al., 2019) suggest the particular importance of carbon for explaining core properties, yet knowledge of carbon partitioning between the outer and inner core is unknown. By using the quasiharmonic approximation, ab initio molecular dynamics, and thermodynamic integration techniques, we have computed the chemical potential of carbon in liquid Fe and solid hcp-Fe at core conditions. We find that substitutional carbon is more stable than interstitial carbon and other carbon defect cluster structures in solid Fe. Lattice strain and overcoordination effects lead to a high chemical potential of C in solid Fe compared to the liquid, and consequently carbon partitions almost completely into the liquid. We find that carbon can account for most of the density jump at the inner-core boundary. This provides an alternative mechanism to the necessity of an oxygen-rich outer core and may have significant implications for the composition and structure of the deep Earth. more...
- Published
- 2019
- Full Text
- View/download PDF
16. The ab initio simulation of the Earth's core
- Author
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Michael J. Gillan, Geoffrey D. Price, Dario Alfè, John P. Brodholt, Lidunka Vočadlo, Alfe, D, Gillan, Mj, Vocadlo, L, Brodholt, J, and Price, Gd
- Subjects
Models, Molecular ,Geologic Sediments ,Silicon ,Hot Temperature ,Earth, Planet ,Iron ,General Mathematics ,Molecular Conformation ,Analytical chemistry ,Ab initio ,General Physics and Astronomy ,Crystal structure ,symbols.namesake ,Thermal ,Pressure ,Computer Simulation ,General Engineering ,Inner core ,Close-packing of equal spheres ,Geology ,Gibbs free energy ,Oxygen ,Core (optical fiber) ,Models, Chemical ,symbols ,Quantum Theory ,Evolution, Planetary ,Sulfur ,Earth (classical element) - Abstract
The Earth has a liquid outer and solid inner core. It is predominantly composed of Fe, alloyed with small amounts of light elements, such as S, O and Si. The detailed chemical and thermal structure of the core is poorly constrained, and it is difficult to perform experiments to establish the properties of core-forming phases at the pressures (ca. 300 GPa) and temperatures (ca. 5000-6000 K) to be found in the core. Here we present some major advances that have been made in using quantum mechanical methods to simulate the high-P/T properties of Fe alloys, which have been made possible by recent developments in high-performance computing. Specifically, we outline how we have calculated the Gibbs free energies of the crystalline and liquid forms of Fe alloys, and so conclude that the inner core of the Earth is composed of hexagonal close packed Fe containing ca. 8.5% S (or Si) and 0.2% O in equilibrium at 5600 K at the boundary between the inner and outer cores with a liquid Fe containing ca. 10% S (or Si) and 8% O. more...
- Published
- 2002
- Full Text
- View/download PDF
17. Ab initio calculations on the free energy and high P-T elasticity of face-centred-cubic iron
- Author
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Lidunka Vočadlo, Geoffrey D. Price, Dario Alfè, Ian G. Wood, Vocadlo, L, Wood, Ig, Alfe, D, and Price, Gd
- Subjects
Alloy ,Ab initio ,Inner core ,chemistry.chemical_element ,engineering.material ,Molecular physics ,Nickel ,Crystallography ,Geophysics ,chemistry ,Space and Planetary Science ,Geochemistry and Petrology ,Ab initio quantum chemistry methods ,Earth and Planetary Sciences (miscellaneous) ,engineering ,Elasticity (economics) ,Fermi gas ,Anisotropy ,Geology - Abstract
Ab initio finite temperature molecular dynamics simulations have been used to calculate the free energy and elasticity of face-centred cubic (fcc) iron at a state point representative of the Earth's inner core. Whilst the free energy of this phase is found to be higher than that of hexagonal-close-packed (hcp) iron, the difference is only 14 meV/atom. It is possible that this difference might be overcome by the presence of light elements, as previous calculations at zero Kelvin have shown that the addition of elements such as silicon stabilise fec-Fe with respect to hcp-Fe by at least 40 meV/atom. The calculated elastic constants at core pressures and temperatures of pure fec-Fe, and of alloys of Fe with sulphur and nickel (Fe(3)S and Fe(3)Ni) derived from the fee structure, lead to average shear wave velocities that are considerably higher than those inferred from seismology; however, these mineralogical and seismological results could be reconciled by the presence of partial melt in the inner core. The calculated P-wave anisotropy of fec-Fe is comparable with the seismological values, but only if there is a high degree of crystal alignment, although the necessity for alignment can be reduced if a layered model for the inner core is invoked. The results presented in this paper therefore suggest that fcc-Fe cannot be ruled out as a candidate for the dominant phase of the Earth's inner core. (c) 2008 Elsevier B.V. All rights reserved. more...
- Published
- 2008
18. Ab initio study of the phase separation of argon in molten iron at high pressures
- Author
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Geoffrey D. Price, Dario Alfè, S. Ostanin, Lidunka Vočadlo, John P. Brodholt, David P. Dobson, Ostanin, S, Alfe, D, Dobson, D, Vocadlo, L, Brodholt, Jp, and Price, Gd
- Subjects
Fusion ,Argon ,Materials science ,Infrared ,Ab initio ,Analytical chemistry ,chemistry.chemical_element ,Molecular dynamics ,Geophysics ,chemistry ,Ab initio quantum chemistry methods ,General Earth and Planetary Sciences ,Physical chemistry ,Solubility ,Eutectic system - Abstract
[1] Using first-principles molecular dynamics (MD) simulations, we study the solubility of argon in molten iron at high pressures and temperatures. In particular we explore whether the low pressure immiscibility of liquid Fe and Ar persists to high pressure (130 GPa) and temperature (4500K), or whether they mix. Starting from a variety of Fe/Ar mixtures we find that they always separate rapidly into two liquids. We conclude that there is no evidence for a significant increase in the solubility of Ar in Fe at these conditions. We cannot, therefore, attribute the lower melting temperatures of Fe obtained from DAC experiments compared to those obtained from ab initio calculations and shock experiments, to eutectic melting between Fe and the Ar pressure medium. more...
- Published
- 2006
19. Computational and experimental studies of solids in the ammonia-water system
- Author
-
Fortes, A., Vocadlo, L, Brodholt, JP, and Wood, IG
- Abstract
This thesis reports the results of first-principles computational studies of thirteen crystalline structures in the H2O-NH3 system. This includes eight low- and highpressure polymorphs of pure water ice, two polymorphs of solid ammonia, and three low-pressure stoichiometric ammonia hydrates. These simulations have been used to determine the athermal equation of state (EoS) of each phase. Where empirical data was lacking, experiments have been undertaken. Hence, this thesis also reports the results of time-of-flight neutron scattering studies of deuterated ammonia dihydrate powders down to 4 K, and up to a maximum pressure of 8.6 GPa. In addition, I have developed a flexible and accurate planetary model that can be used to calculate the triaxial shape and gravitational field of any object, regardless of size or composition, given an assumed mineralogical constitution and provided the EoS of said minerals are known. The EoS parameters found in this work have therefore been used to model the structure and thermal evolution of icy moons orbiting Saturn in anticipation of the Cassini spacecraft arriving at Saturn in mid-2004. Models of Rhea, Saturn’s second largest moon, suggest that its volatile component is likely to contain > 3 weight percent ammonia, but that one is unlikely to be able to constrain the bulk chemistry of the ice mantle from Cassini flyby data. more...
- Published
- 2004
20. Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core
- Author
-
Michael J. Gillan, G. David Price, John P. Brodholt, Ian G. Wood, Lidunka Vočadlo, Dario Alfè, Vocadlo, L, Alfe, D, Gillan, Mj, Wood, Ig, Brodholt, Jp, and Price, Gd
- Subjects
Multidisciplinary ,Silicon ,Chemistry ,Alloy ,Close-packing of equal spheres ,Inner core ,chemistry.chemical_element ,Crystal structure ,engineering.material ,Impurity ,Ab initio quantum chemistry methods ,Chemical physics ,Thermal ,engineering ,Physical chemistry - Abstract
The nature of the stable phase of iron in the Earth's solid inner core is still highly controversial. Laboratory experiments 1 suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations(2-4) have indicated the possible presence of complex, inner-core layering. Theoretical studies(5,6) currently suggest that the hexagonal close packed (h. c. p.) phase of iron is stable at core pressures and that the body centred cubic (b. c. c.) phase of iron becomes elastically unstable at high pressure. In other h. c. p. metals, however, a high-pressure b. c. c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b. c. c. phase will be stabilized with respect to the h. c. p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core(2-4). more...
- Published
- 2002
21. Ab initiomelting curve of the fcc phase of aluminum
- Author
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Dario Alfè, Lidunka Vočadlo, Vocadlo, L, and Alfe, D
- Subjects
Condensed Matter::Materials Science ,Molecular dynamics ,Materials science ,Condensed matter physics ,Dispersion relation ,Phase (matter) ,Anharmonicity ,Ab initio ,Thermodynamics ,Thermodynamic integration ,Density functional theory ,Fermi gas - Abstract
The melting curve of the face-centered cubic (fcc) phase of aluminum has been determined from 0 to $\ensuremath{\sim}150 \mathrm{GPa}$ using first-principles calculations of the free energies of both the solid and liquid. The calculations are based on density functional theory within the generalized gradient approximation using ultrasoft Vanderbilt pseudopotentials. The free energy of the harmonic solid has been calculated within the quasiharmonic approximation using the small-displacement method; the free energy of the liquid and the anharmonic correction to the free energy of the solid have been calculated via thermodynamic integration from suitable reference systems, with thermal averages calculated using ab initio molecular dynamics. The resulting melting curve is in good agreement with both static compression measurements and shock data. more...
- Published
- 2002
- Full Text
- View/download PDF
22. Phonon density of states of iron up to 153 gigapascals
- Author
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Geoffrey D. Price, H. Giefers, Markus Schwoerer-Böhning, Guoyin Shen, Gerhard Wortmann, Daniel Häusermann, Lidunka Vočadlo, Wolfgang Sturhahn, Ho-kwang Mao, R. Lübbers, Peter J. Eng, Dario Alfè, R. J. Hemley, Jian Xu, Michael J. Gillan, Michael Y. Hu, Viktor V. Struzhkin, Jinfu Shu, E. Ercan Alp, Mao, Hk, Xu, J, Struzhkin, Vv, Shu, J, Hemley, Rj, Sturhahn, W, Hu, My, Alp, Ee, Vocadlo, L, Alfe, D, Price, Gd, Gillan, Mj, Schwoerer-Bohning, M, Hausermann, D, Eng, P, Shen, G, Giefers, H, Lubbers, R, and Wortmann, G more...
- Subjects
Bulk modulus ,Multidisciplinary ,Condensed matter physics ,Chemistry ,Phonon ,Inner core ,Inelastic scattering ,Heat capacity ,Physics::Geophysics ,law.invention ,Shear modulus ,symbols.namesake ,law ,symbols ,Preliminary reference Earth model ,Debye model - Abstract
We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy. more...
- Published
- 2001
23. The structure of iron under the conditions of the Earth's inner core
- Author
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Geoffrey D. Price, Dario Alfè, Michael J. Gillan, John P. Brodholt, Lidunka Vočadlo, Vocadlo, L, Brodholt, J, Alfe, D, Price, Gd, and Gillan, Mj
- Subjects
Phase transition ,Seismic anisotropy ,Tetragonal crystal system ,Geophysics ,Materials science ,Polymorphism (materials science) ,Ab initio ,Inner core ,General Earth and Planetary Sciences ,Mineralogy ,Thermodynamics ,Crystal structure ,Anisotropy - Abstract
The inferred density of the solid inner core indicates that it is predominantly made of iron. In order to indicates that it is predominantly made of iron. In order to interpret the observed seismic anisotropy and understand the high pressure and temperature behaviour of the core, it is essential to establish the crystal structure of iron under core conditions. On the basis of extrapolated experimental data, a number of candidate structures for the high PIT iron phase have been proposed, namely, body-centred cubic (bcc), body-centred tetragonal (bct), hexagonal close-packed (hcp), double-hexagonal close-packed (dhcp) and an orthorhombically distorted hcp polymorph (Matsui, 1993; Stixrude and Cohen, 1995; Boehler, 1993; Saxena et al., 1996; Andrault et al., 1997). Here we present the results of the first fully ab initio free energy calculations for all of these polymorphs of iron at core pressures and temperatures. Our results show that hcp-Fe is the most stable polymorph of iron under the conditions of the Earth's inner core. more...
- Published
- 1999
24. Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core-mantle boundary.
- Author
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Dobson DP, Lindsay-Scott A, Hunt SA, Bailey E, Wood IG, Brodholt JP, Vocadlo L, and Wheeler J
- Abstract
The lowermost portion of Earth's mantle (D″) above the core-mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO <subscript>3 postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep. There is excellent agreement between experiments and simulations for both phases in all of the chemical systems studied. Single-crystal diffusivity in postperovskite displays at least 3 orders of magnitude of anisotropy by experiment and simulation ( D
a = 1,000 Db ; Db ≈ Dc ) in zinc fluoride, and an even more extreme anisotropy is predicted ( Da = 10,000 Dc ; Dc = 10,000 Db ) in the natural MgSiO3 system. Anisotropic chemical diffusivity results in anisotropic diffusion creep, texture generation, and a strain-weakening rheology. The results for MgSiO3 postperovskite strongly imply that regions within the D″ region of Earth dominated by postperovskite will 1) be substantially weaker than regions dominated by perovskite and 2) develop a strain-induced crystallographic-preferred orientation with strain-weakening rheology. This leads to strain localization and the possibility to bring regions with significantly varying textures into close proximity by strain on narrow shear zones. Anisotropic diffusion creep therefore provides an attractive alternative explanation for the complexity in observed seismic anisotropy and the rapid lateral changes in seismic velocities in D″. more...- Published
- 2019
- Full Text
- View/download PDF
25. Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core.
- Author
-
Vocadlo L, Alfè D, Gillan MJ, Wood IG, Brodholt JP, and Price GD
- Abstract
The nature of the stable phase of iron in the Earth's solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.c.p.) phase of iron is stable at core pressures and that the body centred cubic (b.c.c.) phase of iron becomes elastically unstable at high pressure. In other h.c.p. metals, however, a high-pressure b.c.c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b.c.c. phase will be stabilized with respect to the h.c.p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core. more...
- Published
- 2003
- Full Text
- View/download PDF
26. Structures and physical properties of epsilon-FeSi-type and CsCl-type RuSi studied by first-principles pseudopotential calculations
- Author
-
Vocadlo L, Price GD, and Wood IG
- Abstract
An investigation of the relative stability of the two known polymorphs of RuSi, having the epsilon-FeSi and CsCl structures, has been made by first-principles pseudopotential calculations. The resulting cell volumes and fractional coordinates at P = 0 are in good agreement with experiment. Application of high pressure to the epsilon-FeSi phase of RuSi is predicted to produce a structure having almost perfect sevenfold coordination. However, it appears that RuSi having the CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 3.6 GPa. Fitting of the calculated internal energy versus volume to a fourth-order logarithmic equation of state led to values (at T = 0 K) for the bulk modulus, K0, of 202 and 244 GPa for the epsilon-FeSi and CsCl phases, respectively, in excellent agreement with experiment. Band-structure calculations for both phases are also presented. more...
- Published
- 2000
- Full Text
- View/download PDF
27. Crystal structure, compressibility and possible phase transitions in \boldvarepsilon-FeSi studied by first-principles pseudopotential calculations.
- Author
-
Vocadlo L, Price GD, and Wood IG
- Abstract
An investigation of the relative stability of the FeSi structure and of some hypothetical polymorphs of FeSi has been made by first-principles pseudopotential calculations. It has been shown that the observed distortion from ideal sevenfold coordination is essential in stabilizing the FeSi structure relative to one of the CsCl type. Application of high pressure to FeSi is predicted to produce a structure having nearly perfect sevenfold coordination. However, it appears that FeSi having a CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 13 GPa. Fitting of the calculated internal energy vs volume for the FeSi structure to a third-order Birch-Murnaghan equation of state led to values, at T = 0 K, for the bulk modulus, K(0), and for its first derivative with respect to pressure, K(0)', of 227 GPa and 3.9, respectively. more...
- Published
- 1999
- Full Text
- View/download PDF
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