| 1. |
- Belonoshko, Anatoly, et al.
(författare)
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Elastic properties of body-centered cubic iron in Earth's inner core
- 2022
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:18
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Tidskriftsartikel (refereegranskat)abstract
- The solid Earth's inner core (IC) is a sphere with a radius of about 1300 km in the center of the Earth. The information about the IC comes mainly from seismic studies. The composition of the IC is obtained by matching the seismic data and properties of candidate phases subjected to high pressure (P) and temperature (T). The close match between the density of the IC and iron suggests that the main constituent of the IC is iron. However, the stable phase of iron is still a subject of debate. One such iron phase, the body-centered cubic phase (bcc), is dynamically unstable at pressures of the IC (330-364 GPa) and low T but gets stabilized at high T characteristic of the IC (5000-7000 K). So far, ab initio molecular dynamics (AIMD) studies attempted to compute the bcc elastic properties for a small (order of 102) number of atoms. The mechanism of the bcc stabilization cannot be enabled in such cells and that has led to erroneous results. Here we apply AIMD to compute elastic moduli and sound velocities of the Fe bcc phase for a 2000 Fe atom computational cell, which is a cell of unprecedented size for ab initio calculations of iron. Unlike in previous ab initio calculations, both the longitudinal and the shear sound velocities of the Fe bcc phase closely match the properties of the IC material at P = 360 GPa and T = 6600 K, likely the PT conditions in the IC. The calculated density of the bcc iron at these PT conditions is just 3% higher than the density of the IC material according to the Preliminary Earth Model. This suggests that the widely assumed amount of light elements in the IC may need a reconsideration. The anisotropy of the bcc phase is an exact match to the most recent seismic studies.
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| 2. |
- Gebresenbut, Girma Hailu, et al.
(författare)
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Superconducting YAu3Si and Antiferromagnetic GdAu3Si with an Interpenetrating Framework Structure Built from 16-Atom Polyhedra
- 2022
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:10, s. 4322-4334
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of reaction mixtures REx(Au0.79Si0.21)100–x (RE = Y and Gd) yielded the compounds REAu3Si which adopt a new structure type, referred to as GdAu3Si structure (tP80, P42/mnm, Z = 16, a = 12.8244(6)/12.7702(2) Å, and c = 9.0883(8)/9.0456(2) Å for GdAu3Si/YAu3Si, respectively). REAu3Si was afforded as millimeter-sized faceted crystal specimens from solution growth employing melts with composition RE18(Au0.79Si0.21)82. In the GdAu3Si structure, the Au and Si atoms are strictly ordered and form a framework built of corner-connected, Si-centered, trigonal prismatic units SiAu6. RE atoms distribute on 3 crystallographically different sites and each attain a 16-atom coordination by 12 Au and 4 Si atoms. These 16-atom polyhedra commonly fill the space of the unit cell. The physical properties of REAu3Si were investigated by heat capacity, electrical resistivity, and magnetometry techniques and are discussed in the light of theoretical predictions. YAu3Si exhibits superconductivity around 1 K, whereas GdAu3Si shows a complex magnetic ordering, likely related to frustrated antiferromagnets exhibiting chiral spin textures. GdAu3Si-type phases with interesting magnetic and transport properties may exist in an extended range of ternary RE–Au–Si systems, similar to the compositionally adjacent cubic 1/1 approximants RE(Au,Si)∼6.
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| 3. |
- Gordeeva, Alisa, et al.
(författare)
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Thermal conversion of the hydrous aluminosilicate LiAlSiO3(OH)2 into γ-eucryptite
- 2021
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Ingår i: Zeitschrift für Naturforschung. B, A journal of chemical sciences. - : Walter de Gruyter GmbH. - 0932-0776 .- 1865-7117. ; 76:10-12, s. 599-606
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Tidskriftsartikel (refereegranskat)abstract
- LiAlSiO3(OH)2 is a dense hydrous aluminosilicate which is formed from LiAlSiO4 glass in hydrothermal environments at pressures around 5 GPa. The OH groups are part of the octahedral Al and Li coordination. We studied the dehydration behavior of LiAlSiO3(OH)2 by a combination of TEM and multi-temperature PXRD experiments. Dehydration takes place in the temperature interval 350–400 °C. Above 700 °C LiAlSiO3(OH)2 is converted via a transient and possibly still slightly hydrous phase into γ-eucryptite which is a metastable and rarely observed polymorph of LiAlSiO4. Its monoclinic structure is built from corner-sharing LiO4, AlO4 and SiO4 tetrahedra. The ordered framework of AlO4 and SiO4 tetrahedra is topologically equivalent to that of cristobalite.
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| 4. |
- Herper, Heike C., et al.
(författare)
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Localized versus itinerant character of 4f-states in cerium oxides
- 2020
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 32:21
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of cerium oxide is investigated here using a combination of ab initio one-electron theory and elements from many-body physics, with emphasis on the nature of the 4f electron shell of cerium ions. We propose to use the hybridization function as a convenient measure for the degree of localization of the 4f shell of this material, and observe that changing the oxidation state is related to distinct changes in the hybridization between the 4f shell and ligand states. The theory reveals that CeO2 has essentially itinerant 4f states, and that in the least oxidized form of ceria, Ce2O3, the 4f states are almost (but not fully) localized. This conclusion is supported by additional calculations based on a combination of density functional theory and dynamical mean field theory. Most importantly, our model points to the fact that diffusion of oxygen vacancies in cerium oxide may be seen as polaron hopping, involving a correlated 4f electron cloud, which is located primarily on Ce ions of several atomic shells surrounding the vacancy.
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| 5. |
- Klarbring, Johan, et al.
(författare)
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Ionic conductivity in Sm-doped ceria from first-principles non-equilibrium molecular dynamics
- 2016
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Ingår i: Solid State Ionics. - : Elsevier. - 0167-2738 .- 1872-7689. ; 296, s. 47-53
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Tidskriftsartikel (refereegranskat)abstract
- Sm-doped ceria is a prospective electrolyte material for intermediate-temperature solid-oxide fuel cells (IT-SOFC). Equi- librium ab initio molecular dynamics (AIMD) studies of oxygen ion diffusion in this material are currently impractical due to the rareness of diffusive events on the accessible timescale. To overcome this issue we have performed ab ini- tio non-equilibrium molecular dynamics calculations of Sm-doped ceria using the color-diffusion algorithm. Applying an external force field we have been able to increase the frequency of diffusive events over the simulation time, while keeping the physical mechanism of diffusion intact. We have investigated the temperature dependence of the maximum strength of the applied external field that could be used while maintaining the response of the system in a linear regime. This allows one to obtain the diffusivity at zero field. The bulk ionic conductivity has been calculated and found to match the experimental data well. We have also compared the description of the diffusion process by our method to previous findings and show that the migration mechanism and site preference of oxygen vacancies with respect to the Sm dopants is well reproduced.
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| 6. |
- Kovacs, Alexander, et al.
(författare)
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Computational Design of Rare-Earth Reduced Permanent Magnets
- 2020
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Ingår i: ENGINEERING. - : ELSEVIER. - 2095-8099 .- 2096-0026. ; 6:2, s. 148-153
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Tidskriftsartikel (refereegranskat)abstract
- Multiscale simulation is a key research tool in the quest for new permanent magnets. Starting with first principles methods, a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition. Iron (Fe)-rich magnetic phases suitable for permanent magnets can be found by means of adaptive genetic algorithms. The intrinsic properties computed by ab intro simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with a realistic structure. Using machine learning techniques, the magnet's structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated. Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases. The following pairs (coercive field (T), energy density product (kJ.m(-3))) were obtained for iron-tin-antimony (Fe3Sn0.75Sb0.25): (0.49, 290), L1(0) -ordered iron-nickel (L1(0) FeNi): (1, 400), cobalt-iron-tantalum (CoFe6Ta): (0.87, 425), and manganese-aluminum (MnAl): (0.53, 80).
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| 7. |
- Lavén, Rasmus, 1994, et al.
(författare)
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Vibrational properties of SrVO2 H with large spin-phonon coupling
- 2022
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Ingår i: Physical Review Materials. - 2475-9953. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- The antiferromagnetic transition metal oxyhydride SrVO2H is distinguished by its stoichiometric composition and an ordered arrangement of H atoms. The tetragonal structure is related to the cubic perovskite and consists of alternating layers of VO2 and SrH. d2 V(III) attains a sixfold coordination by four O and two H atoms. The latter are arranged in a trans fashion, which produces H-V-H chains along the tetragonal axis. Here, we investigate the vibrational properties of SrVO2H by inelastic neutron scattering and infrared spectroscopy combined with phonon calculations based on density functional theory. The H-based vibrational modes divide into a degenerate bending motion perpendicular to the H-V-H chain direction and a highly dispersed stretching motion along the H-V-H chain direction. The bending motion, with a vibrational frequency of approximately 800 cm-1, is split into two components separated by about 50 cm-1, owing to the doubled unit cell from the antiferromagnetic structure. Interestingly, spin-phonon coupling stiffens the H-based modes by 50-100cm-1 although super-exchange coupling via H is very small. Frequency shifts of the same order of magnitude also occur for V-O modes. It is inferred that SrVO2H displays the hitherto largest recognized coupling between magnetism and phonons in a material.
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| 8. |
- Martinez-Casado, R., et al.
(författare)
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The CeFe11Ti permanent magnet : a closer look at the microstructure of the compound
- 2019
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Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 31:50
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Tidskriftsartikel (refereegranskat)abstract
- High-performance permanent magnets (PM) are compounds with outstanding intrinsic magnetic properties. Most PMs are obtained from a favorable combination of rare earth metals (RE = Nd, Pr, Ce) with transition metals (TM = Fe, Co). Amongst them, CeFe11Ti claims considerable attention due to its large Curie temperature, saturation magnetization, and significant magnetocrystalline anisotropic energy. CeFe11Ti has several potential applications, in particular, in the development of electric motors for future automatic electrification. In this work, we shed some light on the mictrostructure of this compound by performing periodic hybrid-exchange density functional theory (DFT) calculations. We use a combined approach of atom-centered local orbitals, plane waves and full-potential linear muffin-tin orbital (LMTO) for our computations. The electronic configuration of the atoms involved in different steps of formation of the crystal structure of CeFe11Ti gives an explanation on the effect of Ce and Ti on its magnetic properties. While Ti stabilizes the structure, atomic orbitals of Ce hybridizes with Fe atomic orbitals to a significant extent and alters the electronic bands. Our calculations confirm a valence of 3(+) for Ce, which has been deemed crucial to obtain a large magnetocrystalline anisotropy. In addition, we analyze several spin configurations, with the ferromagnetic configuration being most stable. We compare and contrast our data to those available and provide an insight for further development of optimized high-performance PMs. Moreover, we compute the Magnetocrystalline Anisotropy of this compound by means of two approaches: the Force Theorem and a full-potential LMTO method.
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| 9. |
- Nieves, P., et al.
(författare)
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Database of novel magnetic materials for high-performance permanent magnet development
- 2019
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 168, s. 188-202
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes the open Novamag database that has been developed for the design of novel Rare-Earth free/lean permanent magnets. Its main features as software technologies, friendly graphical user interface, advanced search mode, plotting tool and available data are explained in detail. Following the philosophy and standards of Materials Genome Initiative, it contains significant results of novel magnetic phases with high magnetocrystalline anisotropy obtained by three computational high-throughput screening approaches based on a crystal structure prediction method using an Adaptive Genetic Algorithm, tetragonally distortion of cubic phases and tuning known phases by doping. Additionally, it also includes theoretical and experimental data about fundamental magnetic material properties such as magnetic moments, magnetocrystalline anisotropy energy, exchange parameters, Curie temperature, domain wall width, exchange stiffness, coercivity and maximum energy product, that can be used in the study and design of new promising high-performance Rare-Earth free/lean permanent magnets. The results therein contained might provide some insights into the ongoing debate about the theoretical performance limits beyond Rare-Earth based magnets. Finally, some general strategies are discussed to design possible experimental routes for exploring most promising theoretical novel materials found in the database.
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| 10. |
- Nilsson, Johan O., et al.
(författare)
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Ionic conductivity in Gd-doped CeO2 : Ab initio color-diffusion nonequilibrium molecular dynamics study
- 2016
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 93:2, s. 024102-
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Tidskriftsartikel (refereegranskat)abstract
- A first-principles nonequilibrium molecular dynamics (NEMD) study employing the color-diffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadolinium-doped cerium oxide (GDC) in the 850-1150 K temperature range. Being a slow process, ionic diffusion in solids usually requires simulation times that are prohibitively long for ab initio equilibrium molecular dynamics. The use of the color-diffusion algorithm allowed us to substantially speed up the oxygen-ion diffusion. The key parameters of the method, such as field direction and strength as well as color-charge distribution, have been investigated and their optimized values for the considered system have been determined. The calculated ionic conductivity and diffusion constants are in good agreement with available experimental data.
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