| 1. |
- Burakovsky, L., et al.
(författare)
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High-Pressure-High-Temperature Polymorphism in Ta : Resolving an Ongoing Experimental Controversy
- 2010
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 104:25, s. 255702-
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Tidskriftsartikel (refereegranskat)abstract
- Phase diagrams of refractory metals remain essentially unknown. Moreover, there is an ongoing controversy over the high-pressure melting temperatures of these metals: results of diamond anvil cell (DAC) and shock wave experiments differ by at least a factor of 2. From an extensive ab initio study on tantalum we discovered that the body-centered cubic phase, its physical phase at ambient conditions, transforms to another solid phase, possibly hexagonal omega phase, at high temperature. Hence the sample motion observed in DAC experiments is very likely not due to melting but internal stresses accompanying a solid-solid transformation, and thermal stresses associated with laser heating.
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| 2. |
- Alling, Björn, et al.
(författare)
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Theoretical investigation of cubic B1-like and corundum (Cr1−xAlx)2O3 solid solutions
- 2013
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 31:3
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations are employed to investigate the stability and properties of cubic rock-salt-like (Cr1−xAlx)2O3 solid solutions, stabilized by metal site vacancies as recently reported experimentally. It is demonstrated that the metal site vacancies can indeed be ordered in a way that gives rise to a suitable fourfold coordination of all O atoms in the lattice. B1-like structures with ordered and disordered metal site vacancies are studied for (Cr0.5Al0.5)2O3 and found to have a cubic lattice spacing close to the values reported experimentally, in contrast to fluorite-like and perovskite structures. The obtained B1-like structures are higher in energy than corundum solid solutions for all compositions, but with an energy offset per atom similar to other metastable systems possible to synthesize with physical vapor deposition techniques. The obtained electronic structures show that the B1-like systems are semiconducting although with smaller band gaps than the corundum structure.
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| 3. |
- 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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| 4. |
- Ekholm, Marcus
(författare)
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Theoretical Descriptions of Complex Magnetism in Transition Metals and Their Alloys
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, various methods for studying solids by simulations of quantummechanical equations, have been applied to transition metals and their alloys. Transition metals such as Fe, Ni, and Mn, are not only cornerstones in modern technology, but also key components in the very fabric of the Earth interior. Such systems show highly complex magnetic properties. As shown within this thesis, to understand and predict their properties from a microscopic level, is still a highly demanding task for the the quantum theory of solids. This is especially crucial at elevated temperature and pressure.It is found that the magnetic degrees of freedom are inseparable from the structural, elastic and chemical properties of such alloy systems. This requires theoretical descriptions capable of handling this interplay. Such schemes are discussed and demonstrated.Furthermore, the importance of the description of Coulomb correlation effects is demonstrated by DFT calculations and also by going beyond the one-electron description by the LDA+DMFT method.It is also shown how magnetic interactions in the half-metallic compound NiMnSb can be manipulated by alloying. The stability of these alloys is also evaluated in calculations, and verified by experimental synthesis at ambient conditions.
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| 5. |
- Ektarawong, Annop, et al.
(författare)
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First-principles study of configurational disorder in B4C using a superatom-special quasirandom structure method
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 90:2
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Tidskriftsartikel (refereegranskat)abstract
- Configurationally disordered crystalline boron carbide, with the composition B4C, is studied using first-principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatoms picture of the complex structure and combine it with a special quasirandom structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: (1) bipolar defects and (2) rotation of icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon an increase in temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites; meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground-state configuration and restore the rhombohedral system (R3m). The restoration of inversion symmetry yielding the full rhombohedral symmetry (R (3) over barm) on average, corresponding to what is reported in the literature, is achieved after the second transition. Investigating the effects of high pressure on the configurational stability of the disordered B4C phases reveals a tendency to stabilize the ordered ground-state configuration as the configurationally ordering/disordering transition temperature increases with pressure exerted on B4C. The electronic density of states, obtained from the disordered phases, indicates a sensitivity of the band gap to the degree of configurational disorder in B4C.
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| 6. |
- Hellman, Olle, et al.
(författare)
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Charge Redistribution Mechanisms of Ceria Reduction
- 2012
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 108:13, s. 135504-
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Tidskriftsartikel (refereegranskat)abstract
- Charge redistribution at low oxygen vacancy concentrations in ceria have been studied in the framework of the density functional theory. We propose a model to approach the dilute limit using the results of supercell calculations. It allows one to reproduce the characteristic experimentally observed behavior of composition versus oxygen pressure dependency. We show that in the dilute limit the charge redistribution is likely to be driven by a mechanism different from the one involving electron localization on cerium atoms. We demonstrate that it can involve charge localization on light element impurities.
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| 7. |
- Hellman, Olle, et al.
(författare)
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Lattice dynamics of anharmonic solids from first principles
- 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. 180301-
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Tidskriftsartikel (refereegranskat)abstract
- An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.
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| 8. |
- Hellman, Olle, et al.
(författare)
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Temperature dependent effective potential method for accurate free energy calculations of solids
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:10
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Tidskriftsartikel (refereegranskat)abstract
- We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in detail. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within ab initio and classical molecular dynamics frameworks. In particular, we examine from first principles the behavior of force constants upon the dynamical stabilization of the body centered phase of Zr, and show that they become more localized. We also calculate the phase diagram for 4He modeled with the Aziz et al. potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.
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| 9. |
- Hellman, Olle, 1981-
(författare)
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Thermal properties of materials from first principles
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the search of clean and efficient energy sources intermediate temperature solid oxide fuel cells are among the prime candidates. What sets the limit of their efficiency is the solid electrolyte. A promising material for the electrolyte is ceria. This thesis aims to improve the characteristics of these electrolytes and help provide thorough physical understanding of the processes involved. This is realised using first principles calculations.The class of methods based on density functional theory generally ignores temperature effects. To accurately describe the intermediate temperature characteristics I have made adjustments to existing frameworks and developed a qualitatively new method. The new technique, the high temperature effective potential method, is a general theory. The validity is proven on a number of model systems.Other subprojects include low-dimensional segregation effects, adjustments to defect concentration formalism and optimisations of ionic conductivity.
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| 10. |
- 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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