| 1. |
- Belonoshko, Anatoly B., et al.
(författare)
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Ab initio equation of state for the body-centered-cubic phase of iron at high pressure and temperature
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:10
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Tidskriftsartikel (refereegranskat)abstract
- The solid inner core of the Earth consists mostly of iron. There is accumulating evidence that, at the extreme pressures and temperatures of the deep Earth interior, iron stabilizes in the body-centered-cubic phase. However, experimental study of iron at those conditions is very difficult at best. We demonstrate that our ab initio approach is capable of providing volumetric data on iron in very good agreement with experiment at low temperature and high pressure. Since our approach treats high-temperature effects explicitly, this allows us to count on similar precision also at high temperature and high pressure. We perform ab initio molecular-dynamics simulations at a number of volume-temperature conditions and compute the corresponding pressures. These points are then fitted with an equation of state. A number of parameters are computed and compared with existing data. The obtained equation of state for high pressure and temperature nonmagnetic body-centered-cubic phase allows the computation of properties of iron under extreme conditions of the Earth's inner core.
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| 2. |
- Belonoshko, Anatoly B., et al.
(författare)
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Elastic anisotropy of Earth's inner core
- 2008
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 319:5864, s. 797-800
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Tidskriftsartikel (refereegranskat)abstract
- Earth's solid- iron inner core is elastically anisotropic. Sound waves propagate faster along Earth's spin axis than in the equatorial plane. This anisotropy has previously been explained by a preferred orientation of the iron alloy hexagonal crystals. However, hexagonal iron becomes increasingly isotropic on increasing temperature at pressures of the inner core and is therefore unlikely to cause the anisotropy. An alternative explanation, supported by diamond anvil cell experiments, is that iron adopts a body- centered cubic form in the inner core. We show, by molecular dynamics simulations, that the body- centered cubic iron phase is extremely anisotropic to sound waves despite its high symmetry. Direct simulations of seismic wave propagation reveal an anisotropy of 12%, a value adequate to explain the anisotropy of the inner core.
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| 3. |
- Belonoshko, Anatoly B., et al.
(författare)
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High-pressure crystal structure studies of Fe, Ru and Os
- 2004
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 65:09-aug, s. 1565-1571
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Tidskriftsartikel (refereegranskat)abstract
- In order to reveal structural trends with increasing pressure in d transition metals, we performed full potential linear muffin-tin orbital calculations for Fe, Ru, and Os in the hexagonal close packed structure. The calculations cover a wide volume range and demonstrate that all these hexagonal close-packed metals have non-ideal c/a at low pressures which, however, increases with pressure and asymptotically approaches the ideal value at very high compressions. These results are in accordance with most recent experiment for Ru and Os. The experimental data for iron is not conclusive, but it is believed that the c/a ratio decreases weakly with increasing pressure at moderate compression. Since, the experimental and calculated equations of state for iron are in increasingly good agreement with increasing pressure, it is possible that either the negative c/a trend is valid only for a restricted pressure range, or related to the experimental difficulties (e.g. non-hydrostaticity).
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| 4. |
- Belonoshko, Anatoly B., et al.
(författare)
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High-pressure melting of MgSiO3
- 2005
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 94:19
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Tidskriftsartikel (refereegranskat)abstract
- The melting curve of MgSiO3 perovskite has been determined by means of ab initio molecular dynamics complemented by effective pair potentials, and a new phenomenological model of melting. Using first principles ground state calculations, we find that the MgSiO3 perovskite phase transforms into post perovskite at pressures above 100 GPa, in agreement with recent theoretical and experimental studies. We find that the melting curve of MgSiO3, being very steep at pressures below 60 GPa, rapidly flattens on increasing pressure. The experimental controversy on the melting of the MgSiO3 perovskite at high pressures is resolved, confirming the data by Zerr and Boehler.
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| 5. |
- Belonoshko, Anatoly B., et al.
(författare)
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High-pressure melting of molybdenum
- 2004
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 92:19
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Tidskriftsartikel (refereegranskat)abstract
- The melting curve of the body-centered cubic (bcc) phase of Mo has been determined for a wide pressure range using both direct ab initio molecular dynamics simulations of melting as well as a phenomenological theory of melting. These two methods show very good agreement. The simulations are based on density functional theory within the generalized gradient approximation. Our calculated equation of state of bcc Mo is in excellent agreement with experimental data. However, our melting curve is substantially higher than the one determined in diamond anvil cell experiments up to a pressure of 100 GPa. An explanation is suggested for this discrepancy.
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| 6. |
- Belonoshko, Anatoly B., et al.
(författare)
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Mechanism for the kappa-Al2O3 to the alpha-Al2O3 transition and the stability of kappa-Al2O3 under volume expansion
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:5, s. 3131-3134
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Tidskriftsartikel (refereegranskat)abstract
- The kappa-Al2O3 metastable phase is an important material for producing cutting tools. However, at temperatures above 1000 K the kappa phase transforms into the stable modification alpha-Al2O3. We have investigated mechanisms for this transformation by means of molecular dynamic simulations using pair potentials. We have found that for the temperature range above 1000 K the mean square displacement of the atoms at the free surface changes its behavior drastically. Since, as was calculated, all other possible driving mechanisms of the phase transition such as pressure and/or temperature without a free surface are not sufficient to cause the transition, the free surface is the major factor initiating the unwanted transition. To hinder the transition one has to slow down the diffusion at the free surface. As an alternative to chemical vapor deposition of thin films of kappa-Al2O3 phase at surfaces of cutting tools, it is found that it is thermodynamically possible to obtain kappa-Al2O3 in a stable phase at a volume expansion of the alpha-Al2O3 phase at a negative pressure of about - 40 kbar.
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| 7. |
- Belonoshko, Anatoly B., et al.
(författare)
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Melting and critical superheating
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:1
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Tidskriftsartikel (refereegranskat)abstract
- Two mechanisms of melting are known, heterogeneous, where melting starts at surfaces, and homogeneous, where the liquid nucleates in the bulk crystal. If melting occurs homogeneously, a crystal can be superheated significantly above its melting temperature (T-m). At present, the physical meaning of the limit of superheating (T-LS) is unknown. We demonstrate, by molecular dynamics simulations, that the total energy of a solid at T-LS is equal to the total energy of its liquid at T-m at the same volume. In the high pressure limit T-LS and T-m are connected by the constant k(AB)=ln 2/3 via the relation k(AB)=T-LS/T-m-1.
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| 8. |
- Belonoshko, Anatoly B., et al.
(författare)
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Melting of Fe and Fe0.9375Si0.0625 at Earth's core pressures studied using ab initio molecular dynamics
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:22
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Tidskriftsartikel (refereegranskat)abstract
- The issue of melting of pure iron and iron alloyed with lighter elements at high pressure is critical to the physics of the Earth. The iron melting curve in the relevant pressure range between 3 and 4 Mbar is reasonably well established from the theoretical point of view. However, so far no one attempted a direct atomistic simulation of iron alloyed with light elements. We investigate here the impact of alloying the body-centered cubic (bcc) Fe with Si. We simulate melting of the bcc Fe and Fe0.9375Si0.0625 alloy by ab initio molecular dynamics. The addition of light elements to the hexagonal-close-packed (hcp) iron is known to depress its melting temperature (T-m). We obtain, in marked contrast, that alloying of bcc Fe with Si does not lead to T-m depression; on the contrary, the T-m slightly increases. This suggests that if Si is a typical impurity in the Earth's inner core, then the stable phase in the core is bcc rather than hcp.
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| 9. |
- Belonoshko, Anatoly B., et al.
(författare)
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Molecular dynamics of LiF melting
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:18, s. 11928-11935
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Tidskriftsartikel (refereegranskat)abstract
- We performed molecular-dynamics simulations of the melting and/or freezing of LiF. The simulations were done using the Tosi-Fumi model and our own model of interatomic interactions. The latter was verified by ab initio calculations of the equation of state for LiF. We show that the recent molecular-dynamics calculations by Boehler and co-workers are not adequate and their model for the interactions is not capable of providing melting temperatures in agreement with experiment. Our calculated pressure dependence of the melting temperatures gives valuable information. We found that the B1-B2 transition in LiF at around 1 Mbar removes the discrepancy between the diamond-anvil cell and shockwave melting temperatures. An explanation of the controversy between low and high melting temperatures obtained from diamond-anvil cell experiments is suggested.
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| 10. |
- Belonoshko, Anatoly B., et al.
(författare)
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Molecular dynamics simulation of the structure of yttria Y2O3 phases using pairwise interactions
- 2001
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 6418:18
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the structure of yttria (Y2O3) by means of ab initio and molecular dynamics methods. The suggested simple model for the interatomic interaction is shown to produce reasonable results at moderate pressures for a wide range of temperatures. The calculated x-ray structure factor is in good agreement with experimental data obtained by the x-ray levitation technique at the temperature of 2526 K. The quality of the agreement decreases with increasing temperature. We demonstrate that it is not necessary to assume nonstoichiometry of liquid yttria, as was done in a recent publication, to obtain agreement with experiment. The structure of liquid yttria can be considered as a mixture of 4- and 6-coordinated Y atoms. We also show the possibility of a light amorphous yttria phase, which possibly can be obtained by quenching from a vapor instead of conventional amorphous yttria quenched from a liquid.
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