| 1. |
- Belonoshko, Anatoly B., et al.
(author)
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A possible mechanism of copper corrosion in anoxic water
- 2012
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In: Philosophical Magazine. - : Informa UK Limited. - 1478-6435 .- 1478-6443. ; 92:36, s. 4618-4627
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Journal article (peer-reviewed)abstract
- Recent experiments show that solid Cu reacts with anoxic water. The reaction is observed by measuring the hydrogen release. This release is continuous and stable over a period of months. We have since theoretically found that water adsorbs dissociatively at a copper surface. But this adsorption is not enough to explain the amount of hydrogen released in the experiment. This observation calls for the explanation of the removal of the reaction product from the surface to provide a clean Cu surface where the water dissociation takes place. In this paper we investigate, by first-principles calculations, two possible mechanisms for this removal: first the possibility of Cu-O-H nanoparticulate formation, and second the diffusion of the dissociation products into Cu. We show that while the formation of nanoparticulates is energetically unfavorable, the diffusion of OH along grain boundaries can be substantial. The OH being placed in a grain boundary of the Cu sample quickly dissociates and O and H atoms diffuse independently of each other. Such a diffusion is markedly larger than the diffusion in bulk Cu. Thus, grain boundary diffusion is a viable mechanism for providing a clean Cu surface for the dissociation of water at the Cu surface. An order-of-magnitude estimate of the amount of hydrogen released in this case agrees with experiment. But this mechanism is not enough to explain the result of the experiment. We propose the formation of nanocrystals of copper oxide as a second step. A decisive experiment is proposed.
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| 2. |
- Belonoshko, Anatoly B., et al.
(author)
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Atomic Diffusion in Solid Molecular Hydrogen
- 2013
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In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 3, s. 2340-
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Journal article (peer-reviewed)abstract
- We performed ab initio molecular dynamics simulations of the C2c and Cmca-12 phases of hydrogen at pressures from 210 to 350 GPa. These phases were predicted to be stable at 0 K and pressures above 200 GPa. However, systematic studies of temperature impact on properties of these phases have not been performed so far. Filling this gap, we observed that on temperature increase diffusion sets in the Cmca-12 phase, being absent in C2c. We explored the mechanism of diffusion and computed melting curve of hydrogen at extreme pressures. The results suggest that the recent experiments claiming conductive hydrogen at the pressure around 260 GPa and ambient temperature might be explained by the diffusion. The diffusion might also be the reason for the difference in Raman spectra obtained in recent experiments.
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| 3. |
- Belonoshko, Anatoly B.
(author)
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Equation of state for epsilon-iron at high pressures and temperatures
- 2010
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In: Condensed Matter Physics. - 1607-324X .- 2224-9079. ; 13:2, s. 23605-23615
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Journal article (peer-reviewed)abstract
- The equation of state for hexagonal close packed (hcp or ∈) phase of Fe at high pressure is created by employing molecular dynamics (MD) simulations in conjunction with the embedded atom method based on the full potential linear muffin tin orbital (FPLMTO) method. Comparison between the existing experimental data and our calculations suggests that the obtained equation of state can be reliably used for calculating iron volumetric properties under conditions appropriate for the Earth's core. We demonstrate that some experimental data on iron might be subjected to a systematic error. I suggest a model which describes the temperature dependence of the volume better than the Mie-Grüneisen equation.
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| 4. |
- Belonoshko, Anatoly B., et al.
(author)
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High-pressure melting curve of platinum from ab initio Z method
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:17, s. 174104-
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Journal article (peer-reviewed)abstract
- Pt is widely used as a standard in high-pressure high-temperature experiments. The available experimental and theoretical data on Pt thermal stability is not consistent. We address the issue of high-pressure Pt melting by ab initio molecular dynamics. We demonstrate a remarkable consistency of our computed melting curve with the experimental data by N. R. Mitra, D. L. Decker, and H. B. Vanfleet [Phys. Rev. 161, 613 (1967)]. The extrapolation of their data, based on the Simon equation, nearly coincides with our ab initio computed melting curve. We propose the Pt melting curve in the form P-m(kbar) = 443.0[(T/T-m)(1.14) - 1].
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| 5. |
- Belonoshko, Anatoly B., et al.
(author)
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Melting of a polycrystalline material
- 2013
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In: The European Physical Journal Special Topics. - : Springer Science and Business Media LLC. - 1951-6355 .- 1951-6401. ; 216:1, s. 199-204
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Journal article (peer-reviewed)abstract
- Calculating the melting temperature of a solid with a known model of interaction between atoms is nowadays a comparatively simple task. However, when one simulates a single crystal by molecular dynamics method, it does not normally melt at the melting temperature. Instead, one has to significantly overheat it. Yet, a real material melts at the melting point. Here we investigate the impact of the defects and the grain boundaries on melting. We demonstrate that defects and grain boundaries have similar impact and make it possible to simulate melting in close vicinity of thermodynamic melting temperature. We also show that the Z method might be non-applicable in discriminating a stable submelting phase.
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| 6. |
- Belonoshko, Anatoly B., et al.
(author)
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MgO phase diagram from first principles in a wide pressure-temperature range
- 2010
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:5, s. 054110-1-054110-9
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Journal article (peer-reviewed)abstract
- Recent laser-initiated strong shockwave measurements at Livermore provide the opportunity for verification of the MgO phase diagram at extreme pressures and temperatures. This calls for a comprehensive study of the MgO phase diagram. The phase diagram is obtained by ab initio molecular dynamics (two phase and Z method) and phonon-based thermodynamic calculations. Energies and forces are computed from first principles projector augmented wave method. The B1 transforms to B2 phase at about 490 GPa. Melting temperatures of B1 are consistent with the two-phase melting curve (Alfe, 2005). The triple point B1-B2-liquid is located at about 2.4 Mbar and 9000 K. The melting curve of the B2 phase rises rather steeply from the triple point. The Hugoniot is likely to cross the B1-B2 boundary first and then to cross the melting curve of B2, therefore, the melting curve of periclase is not relevant for the Hugoniot. MgO melts between 11.3 and 12.5 thousand K and 4.3 and 5 Mbar along the Hugoniot from the B2 phase. The two-phase melting curves of B1 computed with various semiempirical potentials are remarkably close to each other and are consistent with the B1 first principles melting curve at low pressure. This suggests the MgO melting temperatures to be in the close proximity of this determination. The search for new phases of MgO by first principles metadynamics has not produced unknown phases. Therefore, the suggested discontinuity of the Hugoniot at 170 GPa and 3000 K remains enigmatic.
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| 7. |
- Belonoshko, Anatoly B., et al.
(author)
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Shear Relaxation in Iron under the Conditions of Earth's Inner Core
- 2010
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 104:24, s. 245703-
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Journal article (peer-reviewed)abstract
- Large scale molecular dynamics simulations of iron at high pressure and temperature are performed to investigate the physics of shear softening. A solid 16 x 10(6) atoms sample of iron is grown out of the liquid with a small solid immersed in it at the start of simulation. We observe that diffusion in the sheared solid is similar to that in liquid, even though at different time scales. This allows us to describe the time dependence of shear stress in terms of elastic and hydrodynamic relaxation. The elastic response of the sample is close to the elastic response of Earth's inner core. This explains the abnormally low shear modulus in the core. The reason for the low shear modulus is the presence of defects of the crystal structure.
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| 8. |
- Benazzouz, Brahim K., et al.
(author)
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Determination of the melting temperature of kaolinite by means of the Z-method
- 2013
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In: American Mineralogist. - : Mineralogical Society of America. - 0003-004X .- 1945-3027. ; 98:10, s. 1881-1885
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Journal article (peer-reviewed)abstract
- The melting temperature of materials is an important thermodynamic property. Despite the importance of kaolinite, one of the most common clay minerals on the Earth's surface, its thermal and melting behavior is poorly understood. We apply here the Z-method to determine the melting temperature (T-m) and the limit of superheating (T-LS) of kaolinite. The T-m is found at 1818 K (8.85 GPa), and T-LS at 1971 K (6.8 GPa). The diffusion coefficient for all atoms has been calculated in a broad temperature range. The calculated characteristics and, in particular, their dependence on temperature have confirmed the solid-liquid transition and strongly support the calculated melting point. In addition, some computed quantities, such as the radial distribution function, coordination numbers and mean-square displacement, were used to confirm the liquid state of kaolinite from the melting temperature as well as at other temperatures in the liquid branch. The diffusion coefficient for different atoms has been calculated throughout the isochore. These quantities and in particular their evolution under temperature have confirmed the solid-liquid states of kaolinite and the presence of the melting point. The latter quantity constitutes the first ever melting simulation of a clay mineral with close agreement to the experimental one.
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| 9. |
- Bryk, T., et al.
(author)
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Collective excitations in molten iron above the melting point : A generalized collective-mode analysis of simulations with embedded-atom potentials
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:2, s. 024202-
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Journal article (peer-reviewed)abstract
- It is shown, that the embedded-atom potential nicely describing structural properties of high pressure Fe can be successfully used for description of collective dynamics of liquid iron. A combination of molecular dynamics simulations and a fit-free analysis based on the approach of generalized collective modes (GCM) is used for calculations of spectra of collective excitations and relaxing modes at 1843 K. The obtained spectrum of acoustic excitations in the long-wavelength region perfectly agrees with the experimental speed of sound and reproduces the dispersion estimated from inelastic X-ray scattering (IXS) experiments. Heat fluctuations in liquid Fe were studied and resulted in calculated ratio of specific heats γ-1.40 being in agreement with the IXS-experiment estimate. We report analysis of the wave-number dependence of relaxation processes and their contributions to dynamic structure factors. This permits estimation of most important relaxation processes contributing to the shape of dynamic structure factors of liquid Fe in different regions of wave numbers.
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| 10. |
- Davis, Sergio, et al.
(author)
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Model for diffusion at the microcanonical superheating limit from atomistic computer simulations
- 2011
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:6, s. 064102-
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Journal article (peer-reviewed)abstract
- The diffusion statistics of atoms in a crystal close to the critical superheating temperature was studied in detail using molecular dynamics and Monte Carlo simulations. We present a continuous random-walk model for diffusion of atoms hopping through thermal vacancies. The results obtained from our model suggest that the limit of superheating is precisely the temperature for which dynamic percolation happens at the time scale of a single individual jump. A possible connection between the critical superheating limit and the maximization of the Shannon entropy associated with the distribution of jumps is suggested. As a practical application of our results, we show that an extrapolation of the critical superheating temperature (and therefore an estimation of the melting point) can be performed using only the dynamical properties of the solid state.
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| 11. |
- Dewaele, A., et al.
(author)
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High-pressure high-temperature equation of state of KCl and KBr
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:21, s. 214105-
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Journal article (peer-reviewed)abstract
- The equation of state of KCl and KBr, compressed in a helium pressure medium in a diamond-anvil cell, has been measured by x-ray diffraction in the B1 and B2 phases up to 165 GPa at 298 K. The P-V-T of B2 KCl and B2 KBr has been calculated by ab initio molecular dynamics in a wide compression range and up to 7000 K. The thermal pressure exhibits a linear behavior with temperature and remains low under high compression. The experimental P-V points and the thermal pressure calculated by molecular dynamics have been used to set up a high-pressure high-temperature equation of state of B2 KCl and B2 KBr. With these equations of state, B2 KCl and B2 KBr can be used as pressure markers in laser-heated diamond-anvil-cell experiments.
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| 12. |
- Lukinov, Tymofiy, et al.
(author)
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Impact of crystal lattice defects on crystal melting : A molecular dynamics study
- 2013
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In: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 79, s. 95-98
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Journal article (peer-reviewed)abstract
- An ideal infinite lattice, when subjected to heating does not melt at the thermodynamic melting temperature. Instead, it remains solid metastably up to considerably higher temperatures. This effect is called superheating. We performed a molecular dynamics simulation of Xenon using Lennard-Jonesium potential with several types of defects. We observed a superheating effect on the chosen model for several pressures and found that the presence of grain inclusion or grain interface eliminates the superheating effect.
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| 13. |
- Mattesini, Maurizio, et al.
(author)
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Hemispherical anisotropic patterns of the Earth's inner core
- 2010
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In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 107:21, s. 9507-9512
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Journal article (peer-reviewed)abstract
- It has been shown that the Earth's inner core has an axisymmetric anisotropic structure with seismic waves traveling similar to 3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth's core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres. The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth's rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth's inner core.
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| 14. |
- Ruban, Andrei V., et al.
(author)
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Impact of magnetism on Fe under Earth's core conditions
- 2013
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:1, s. 014405-
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Journal article (peer-reviewed)abstract
- Using a microscopic phenomenological model for longitudinal spin fluctuations (LSFs) based on density functional theory calculations, we demonstrate that under the Earth's core conditions (P approximate to 360 GPa, T approximate to 6000 K), Fe acquires substantial local magnetic moment, up to 1.3 mu(B), for different crystal structure modifications. We demonstrate that the LSFs produce a substantial effect on the magnetic and thermodynamic properties of iron, in particular, its equilibrium volume under solid Earth's core conditions.
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