Publications (124)515.58 Total impact
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ABSTRACT: A firstprinciples nonequilibrium molecular dynamics (NEMD) study employing the colordiffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadoliniumdoped 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 colordiffusion algorithm allowed us to substantially speed up the oxygenion diffusion. The key parameters of the method, such as field direction and strength as well as colorcharge 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.  [Show abstract] [Hide abstract]
ABSTRACT: Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide B$_{13}$C$_{2}$ has been a controversial topic in the field of icosahedral boronrich solids. We demonstrate that this discrepancy is removed when configurational disorder is accurately considered in the theoretical calculations. We find that while ordered ground state B$_{13}$C$_{2}$ is metallic, configurationally disordered B$_{13}$C$_{2}$, modeled with a superatomspecial quasirandom structure method, goes through a metal to nonmetal transition as the degree of disorder is increased with increasing temperature. Specifically, one of the chainend carbon atoms in the CBC chains substitutes a neighboring equatorial boron atom in a B$_{12}$ icosahedron bonded to it, giving rise to a B$_{11}$C$^{e}$(BBC) unit. The atomic configuration of the substitutionally disordered B$_{13}$C$_{2}$ thus tends to be dominated by a mixture between B$_{12}$(CBC) and B$_{11}$C$^{e}$(BBC). Due to splitting of valence states in B$_{11}$C$^{e}$(BBC), the electron deficiency in B$_{12}$(CBC) is gradually compensated.  [Show abstract] [Hide abstract]
ABSTRACT: The sound velocity of Mo along the Hugoniot adiabat is calculated from first principles using densityfunctional theory based molecular dynamics. These data are compared to the sound velocity as measured in recent experiments. The theoretical and experimental Hugoniot and sound velocities are in very good agreement up to pressures of 210 GPa and temperatures of 3700 K on the Hugoniot. However, above that point the experiment and theory diverge. This implies that Mo undergoes a phase transition at about the same point. Considering that the melting point of Mo is likely much higher at that pressure, the related change in the sound velocity in experiment can be ascribed to a solidsolid transition.  [Show abstract] [Hide abstract]
ABSTRACT: We have performed a numerical study of the process of electron localization in reduced ceria. Our results show that different localized charge distributions can be attained in a bulk system by varying the lattice parameter. We demonstrate that the effect of electron localization is mainly determined by lattice relaxation and an accurate account for the effects of electronic correlation is necessary to achieve localized charge distribution. 
Article: Electronic correlations in Fe at Earth's inner core conditions: Effects of alloying with Ni
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ABSTRACT: We have studied the bodycentered cubic (bcc), facecentered cubic (fcc) and hexagonal closepacked (hcp) phases of Fe alloyed with 25 at. % of Ni at Earth's core conditions using an ab initio local density approximation + dynamical meanfield theory (LDA+DMFT) approach. The alloys have been modeled by ordered crystal structures based on the bcc, fcc, and hcp unit cells with minimum possible cell size allowing for the proper composition. Our calculations demonstrate that the strength of electronic correlations on the Fe 3d shell is highly sensitive to the phase and local environment. In the bcc phase the 3d electrons at the Fe site with Fe only nearest neighbors remain rather strongly correlated even at extreme pressuretemperature conditions, with the local and uniform magnetic susceptibility exhibiting a CurieWeisslike temperature evolution and the quasiparticle lifetime {\Gamma} featuring a nonFermiliquid temperature dependence. In contrast, for the corresponding Fe site in the hcp phase we predict a weaklycorrelated Fermiliquid state with a temperatureindependent local susceptibility and a quadratic temperature dependence of {\Gamma}. The iron sites with nickel atoms in the local environment exhibit behavior in the range between those two extreme cases, with the strength of correlations gradually increasing along the hcpfccbcc sequence. Further, the intersite magnetic interactions in the bcc and hcp phases are also strongly affected by the presence of Ni nearest neighbors. The sensitivity to the local environment is related to modifications of the Fe partial density of states due to mixing with Ni 3dstates.  [Show abstract] [Hide abstract]
ABSTRACT: Configurationally disordered crystalline boron carbide, B$_{4}$C, is studied using firstprinciples calculations. We investigate both dilute and high concentrations of carbonboron substitutional defects. For the latter purpose, we suggest a superatom's 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 lowenergy defects: 1) Bipolar defects and 2) Rotation of icosahedral carbon among the three polarup 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 increasing temperature using a meanfield approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polarup 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 (R3m) on average, corresponding to what is reported in the literature, is achieved after the second transition. The electronic density of states, obtained from the disordered phases indicates a sensitivity of band gap to the degree of configurational disorder in B$_{4}$C.  [Show abstract] [Hide abstract]
ABSTRACT: Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft OH⋯O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal xray diffraction, Mössbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the Hbonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe^{3+} hightolow spin crossover at above 45 GPa.  [Show abstract] [Hide abstract]
ABSTRACT: Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mössbauer transition of ^{61}Ni. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the LambMössbauer factor increases from 33 meV at ambient pressure to 60 meV at 100 GPa. The change of this energy over volume compression is well described by a Grüneisen parameter of 2.09. 
Article: Temperature dependent effective potential method for accurate free energy calculations of solids
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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 \emph{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 details. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within \emph{ab initio} and classical molecular dynamics frameworks. In particular, we examine from firstprinciples the behavior of force constants upon the dynamical stabilization of body centered phase of Zr, and show that they become more localized. We also calculate phase diagram for $^4$He modeled with the Aziz \emph{et al.} potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.  [Show abstract] [Hide abstract]
ABSTRACT: We review basic ideas behind stateoftheart techniques for firstprinciples theoretical simulations of the phase stabilities and properties of alloys. We concentrate on methods that allow for an efficient treatment of compositional and thermal disorder effects. In particular, we present novel approach to evaluate free energy for strongly anharmonic systems. Theoretical tools are then employed in studies of two materials systems relevant for nuclear energy applications: FeCr and Zrbased alloys. In particular, we investigate the effect of hydrostatic pressure and multicomponent alloying on the mixing enthalpy of FeCr alloys, and show that in the ferromagnetic state both of them reduce the alloy stability at low Cr concentration. For ZrNb alloys, we demonstrate how microscopic parameters calculated from firstprinciples can be used in higherlevel models.  [Show abstract] [Hide abstract]
ABSTRACT: The lattice stability trends of the primary candidate for Earth's core material, the FeNi alloy, were examined from first principles. We employed the exact muffintin orbital method (EMTO) combined with the coherent potential approximation (CPA) for the treatment of alloying effects. It was revealed that high pressure reverses the trend in the relative stabilities of the bodycentered cubic (bcc), facecentered cubic (fcc), and hexagonal closepacked (hcp) phases observed at ambient conditions. In the low pressure region the increase of Ni concentration in the FeNi alloy enhances the bcc phase destabilization relative to the more closepacked fcc and hcp phases. However, at 300 GPa (Earth's core pressure), the effect of Ni addition is opposite. The reverse of the trend is associated with the suppression of the ferromagnetism of Fe when going from ambient pressures to pressure conditions corresponding to those of Earth's core. The firstprinciples results are explained in the framework of the canonical band model.  [Show abstract] [Hide abstract]
ABSTRACT: We employ stateoftheart ab initio simulations within the dynamical meanfield theory to study three likely phases of iron (hexogonal closepacked, hcp, face centered cubic, fcc, and body centered cubic, bcc) at the Earth's core conditions. We demonstrate that the correction to the electronic free energy due to correlations can be significant for the relative stability of the phases. The strongest effect is observed in bcc Fe, which shows a nonFermi liquid behaviour, and where a CurieWeiss behaviour of the uniform susceptbility hints at a local magnetic moment still existing at 5800 K and 300 GPa. We predict that all three structures have sufficiently high magnetic susceptibility to stabilize the geodynamo.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: Herein, we use first principles calculations to study the energy of the (112̅ 1) twin boundary in Zr, Zn, Mg, Ti, and Be. This boundary is important for understanding the microyielding and damping of hexagonal closepacked metals. The (112̅ 1) 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 secondorder elastic constant term, c44, 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 (112̅ 1) twin, are so low as to be below the threshold of the first principles calculations.  [Show abstract] [Hide abstract]
ABSTRACT: The effect of hydrostatic pressure on the phase stability of FeCr alloys has been studied using ab initio methods. We show that while pressure decreases the tendency toward the phase separation in the paramagnetic state of bcc alloys, in the ferromagnetic state it reduces the alloy stability at low Cr concentration and vice versa, makes the solid solution more stable at higher concentrations. This behavior of the phase stability can be predicted from the deviation of the lattice parameter from Vegard’s law in bcc FeCr alloys. On the atomic level, the pressure effect can be explained by the suppression of the local magnetic moments on Cr atoms, which gives rise to a decrease of the FeCr magnetic exchange interaction at the first coordination shell and, as a result, to the observed variation of the ordering tendency between the Fe and Cr atoms. 
Conference Paper: Electrondeficient and polycenter bonds in γB 28
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ABSTRACT: By means of abinitio 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 Earth's core. We show from abinitio calculations that the ordering energies of fcc and hcpstructured 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 120–360GPa and 1000–6600K. Our calculations show that alloying of Fe with Ni below 3000K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000K the effect is reversed, and at conditions corresponding to those of the Earth's inner core, Ni acts as an agent to stabilise the hcp phase.  [Show abstract] [Hide abstract]
ABSTRACT: An accurate equation of state (EOS) is determined for the highpressure orthorhombic phase of boron, B28, 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 B28. In theory, we take into consideration the lattice vibrations, often neglected in firstprinciples simulations. We show that the phonon contribution has a profound effect on the EOS of B28, giving rise to anomalously low values of the pressure derivative of the bulk modulus and greatly improving the agreement between theory and experiment.  [Show abstract] [Hide abstract]
ABSTRACT: An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on firstprinciples 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. 
Article: ChemInform Abstract: The Origin of the Distorted ClosePacked Elemental Structure of Indium
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ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Publication Stats
4k  Citations  
515.58  Total Impact Points  
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Institutions

20052015

Linköping University
 Department of Physics, Chemistry and Biology (IFM)
Linköping, Östergötland, Sweden


19982010

Stockholm University
 Department of Organic Chemistry
Tukholma, Stockholm, Sweden


19962005

Uppsala University
 Department of Physics and Astronomy
Uppsala, Uppsala, Sweden


19992003

Chalmers University of Technology
 Department of Applied Physics
Goeteborg, Västra Götaland, Sweden


2002

University of Gothenburg
Goeteborg, Västra Götaland, Sweden


2001

University of Pennsylvania
 Laboratory for Research on the Structure of Matter
Philadelphia, Pennsylvania, United States
