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  • 1. Klarbring, Johan
    et al.
    Vekilova, Olga Yu
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Uppsala University, Sweden.
    Nilsson, Johan O.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Skorodumova, Natalia V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Uppsala University, Sweden.
    Simak, Sergei I.
    Ionic conductivity in Sm-doped ceria from first-principles non-equilibrium molecular dynamics2016Inngår i: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 296, s. 47-53Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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. 

  • 2.
    Nilsson, Johan O.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Leetmaa, Mikael
    Vekilova, Olga Yu
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Oxygen diffusion in ceria doped with rare-earth elementsManuskript (preprint) (Annet vitenskapelig)
  • 3.
    Nilsson, Johan O.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Leetmaa, Mikael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Vekilova, Olga Yu.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden.
    Oxygen diffusion in ceria doped with rare-earth elements2017Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, s. 13723-13730Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We examine the effects of the dopant type and the dopant distribution on the ion diffusion in ceria doped with rare-earth elements (Pr, Nd, Pm, Sm, Eu, and Gd). Diffusion is simulated by means of a Kinetic Monte Carlo method using input transition rates derived from diffusion barriers calculated in the framework of density functional theory (DFT). Based on diffusion simulations, we discuss the characteristics of the dopants in terms of the diffusion barriers, and study oxygen ion trajectories for different dopants and distributions. Our simulations show a trend of increasing ion diffusivity with increasing atomic number for all distributions.

  • 4.
    Nilsson, Johan O.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Leetmaa, Mikael
    Vekilova, Olga Yu
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Uppsala University, Sweden.
    Statistical error in simulations of Poisson processes: Example of diffusion in solids2016Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 94, nr 8, artikkel-id 085206Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Simulations of diffusion in solids often produce poor statistics of diffusion events. We present an analytical expression for the statistical error in ion conductivity obtained in such simulations. The error expression is not restricted to any computational method in particular, but valid in the context of simulation of Poisson processes in general. This analytical error expression is verified numerically for the case of Gd-doped ceria by running a large number of kinetic Monte Carlo calculations. 

  • 5.
    Nilsson, Johan O.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Vekilova, Olga Yu
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hellman, Olle
    Klarbring, Johan
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ionic conductivity in Gd-doped CeO2: Ab initio color-diffusion nonequilibrium molecular dynamics study2016Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, nr 2, artikkel-id 024102Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 6.
    Vekilova, Olga Yu.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Flerskalig materialmodellering. Linkoping Univ, Sweden.
    Pourovskii, L. V.
    Abrikosov, I. A.
    Simak, S. I.
    Electronic correlations in Fe at Earth's inner core conditions: Effects of alloying with Ni2015Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, nr 24, artikkel-id 245116Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have studied the body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) phases of Fe alloyed with 25 at.% of Ni at Earth's core conditions using an ab initio local density approximation + dynamical mean-field theory approach. The alloys have been modeled by ordered crystal structures based on the bcc, fcc, and hcp unit cells with the 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 pressure-temperature conditions, with the local and uniform magnetic susceptibility exhibiting a Curie-Weiss-like temperature evolution and the quasiparticle lifetime Gamma featuring a non-Fermi-liquid temperature dependence. In contrast, for the corresponding Fe site in the hcp phase, we predict a weakly correlated Fermi-liquid state with a temperature-independent 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 hcp-fcc-bcc 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 3d states.

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