| 1. |
- Ekholm, Marcus, et al.
(författare)
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Configurational thermodynamics of Fe-Ni alloys at Earths core conditions
- 2011
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Ingår i: Earth and Planetary Science Letters. - : Elsevier. - 0012-821X .- 1385-013X. ; 308:1-2, s. 90-96
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Tidskriftsartikel (refereegranskat)abstract
- By means of ab-initio calculations, we perform an analysis of the configurational thermodynamics, effects of disorder, and structural energy differences in Fe-Ni alloys at the pressure and temperature conditions of the Earths core. We show from ab-initio calculations that the ordering energies of fcc and hcp-structured Fe-Ni solid solutions at these conditions depend sensitively on the alloy configuration, i.e., on the degree of chemical disorder, and are on a scale comparable with the structural energy differences. From configurational thermodynamic simulations we find that a distribution of Fe and Ni atoms in the solutions should be very close to completely disordered at these conditions. Using this model of the Fe-Ni system, we have calculated the fcc-hcp structural free energy difference in a wide pressure-temperature range of 120-360 GPa and 1000-6600K. Our calculations show that alloying of Fe with Ni below 3000 K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000 K the effect is reversed, and at conditions corresponding to those of the Earths inner core, Ni acts as an agent to stabilise the hcp phase.
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| 2. |
- Isaev, Eyvas, et al.
(författare)
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Impact of lattice vibrations on equation of state of the hardest boron phase
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:13, s. 132106-
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Tidskriftsartikel (refereegranskat)abstract
- An accurate equation of state (EOS) is determined for the high-pressure orthorhombic phase of boron, B(28), experimentally as well as from ab initio calculations. The unique feature of our experiment is that it is carried out on the single crystal of B(28). In theory, we take into consideration the lattice vibrations, often neglected in first-principles simulations. We show that the phonon contribution has a profound effect on the EOS of B(28), giving rise to anomalously low values of the pressure derivative of the bulk modulus and greatly improving the agreement between theory and experiment.
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| 3. |
- Lane, Nina J, et al.
(författare)
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First-principles study of dislocations in hcp metals through the investigation of the (11(2)over-bar1) twin boundary
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:18, s. 184101-
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Tidskriftsartikel (refereegranskat)abstract
- Herein, we use first principles calculations to study the energy of the (11 (2) over bar1) twin boundary in Zr, Zn, Mg, Ti, and Be. This boundary is important for understanding the microyielding and damping of hexagonal close-packed metals. The (11 (2) over bar1) twin boundary is unique in that it is composed of-and can form by the glide of-basal dislocations nucleating at every c lattice parameter. The effect of the number of atoms between boundaries on the boundary energy, and the resulting lattice strains of the relaxed structures are quantified. It is shown that the energies obtained converge within 32-64 atoms/supercell. The structures with a higher second-order elastic constant term, c(44), also have higher boundary energies. It is further shown that the critical resolved shear stresses of the basal dislocations at 0 K, which make up the (11 (2) over bar1) twin, are so low as to be below the threshold of the first principles calculations.
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| 4. |
- Mondal, Swastik, et al.
(författare)
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Electron-Deficient and Polycenter Bonds in the High-Pressure gamma-B-28 Phase of Boron
- 2011
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Ingår i: PHYSICAL REVIEW LETTERS. - : American Physical Society. - 0031-9007. ; 106:21, s. 215502-
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Tidskriftsartikel (refereegranskat)abstract
- The peculiar bonding situation in gamma boron is characterized on the basis of an experimental electron-density distribution which is obtained by multipole refinement against low-temperature single-crystal x-ray diffraction data. A topological analysis of the electron-density distribution reveals one-electron-two-center bonds connecting neighboring icosahedral B-12 clusters. A unique polar-covalent two-electron-three-center bond between a pair of atoms of an icosahedral cluster and one atom of the interstitial B-2 dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.
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