| 1. |
- Bocharov, D., et al.
(författare)
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Electronic structure of cubic ScF3 from first-principles calculations
- 2016
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 42:7, s. 556-560
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Tidskriftsartikel (refereegranskat)abstract
- The ground state properties of cubic scandium trifluoride (ScF3) perovskite were studied using first-principles calculations. The electronic structure of ScF3 was determined by linear combination of atomic orbital (LCAO) and plane wave projector augmented-wave (PAW) methods using modified hybrid exchange-correlation functionals within the density functional theory (DFT). The comprehensive comparison of the results obtained by two methods is presented. Both methods allowed us to reproduce the lattice constant found experimentally in ScF3 at low temperatures and to predict its electronic structure in good agreement with known experimental valence-band photoelectron and F 1s x-ray absorption spectra.
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| 2. |
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| 3. |
- Nilsson, Johan O., et al.
(författare)
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Modeling Kinetics of Water Adsorption on the Rutile TiO2 (110) Surface: Influence of Exchange-Correlation Functional
- 2018
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Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 255:3
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Tidskriftsartikel (refereegranskat)abstract
- The accuracy of the theoretical description of materials properties in the framework of density functional theory (DFT) inherently depends on the exchange-correlation (XC) functional used in the calculations. Here we investigate the influence of the choice of a XC functional (PBE, RPBE, PW91, and PBE0) on the kinetics of the adsorption, diffusion and dissociation of water on the rutile TiO2(110) surface using a combined Kinetic Monte Carlo (KMC) - DFT approach, where the KMC simulations are based on the barriers for the aforementioned processes calculated with DFT. We also test how the adsorption energy of intact and dissociated water molecules changes when dispersion interactions are included into the calculations. We consider the beginning of the water layer formation varying coverage up to 0.2 monolayer (ML) at temperatures up to 180K. We demonstrate that the dynamics of the simulated water-titania system is extremely sensitive to the choice of the XC functional.
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| 4. |
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| 5. |
- Žguns, Pjotrs A.
(författare)
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Configurational Thermodynamics of the CeO2-Gd2O3 System : A Combined DFT, Cluster Expansion and Monte Carlo Approach to Bulk and Interfaces
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, we study the configurational thermodynamics of Ce1-xGdxO2-x/2 x ≤ 1 or CGO. We apply a combined Density Functional Theory (DFT), cluster expansion and Monte Carlo (MC) approach in which the configurational energy of CGO is described by means of the Ising-type Hamiltonian. The interactions are determined by the cluster expansion of total energies calculated with DFT for a set of various cation–anion configurations. This allows one to perform on- the-fly calculations of the configurational energies in MC simulations of cation and anion ordering.The cluster interactions are essentially electrostatic and long-range, and describe the configurational energetics in the entire range of concentrations rather well.The phase diagram obtained in the MC simulations allows one to rationalise existing experimental data and is largely in agreement with that. We observe the phase separation into pure oxides, in equilibrium, below ca. 1000 K for x ≤ 1 (however it is kinetically hindered as diffusion of cations below ca. 1500 K is slow). We also observe the C-type oxygen–vacancy ordering in the x = 0.3–1 range below ca. 1200–3300 K (C phase), and a largely disordered, fluorite (F) phase in the x ≤ 0.3 range and above the ordering temperature.The DFT supercell calculations of the F and C phase configurations obtained in MC simulations allow us to study the effect of concentration and ordering on the lattice relaxations, lattice parameter and elastic moduli, providing insights into relation between preparation conditions, structure and properties.The bulk cluster interactions appear to be applicable also to coherent CeO2/C- type Gd2O3 interfaces, hence we examine a configurational energy landscape of the oxygen vacancy migration therein.This combined approach can be applied to study configurational thermodynamics of similar materials as well as the influence of configurational state on ionic conductivity and other properties.
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| 6. |
- Žguns, Pjotrs A., et al.
(författare)
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Cu adatom charging on Mo supported ScN, MgO and NaF
- 2015
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Ingår i: RSC Advances. - : Royal Society of Chemistry. - 2046-2069. ; 5:114, s. 94436-94445
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Tidskriftsartikel (refereegranskat)abstract
- Molybdenum supported thin films of ScN, MgO and NaF with a Cu adatom have been studied in the framework of density functional theory. We have observed a charge transfer from the metal/film interface to the Cu atom and investigated its relation to surface and interface deformations. We find that a weak interaction between the metal and the film is a promising prerequisite for adatom charging. The detailed study of Cu/NaF/Mo and NaF/Mo indicates that the distortion of the NaF film caused by the Cu adsorption has essentially anharmonic character, as it is coupled to a strong charge redistribution in the system.
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| 7. |
- Zguns, Pjotrs A., et al.
(författare)
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Ordering and phase separation in Gd-doped ceria : a combined DFT, cluster expansion and Monte Carlo study
- 2017
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 19:39, s. 26606-26620
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Tidskriftsartikel (refereegranskat)abstract
- Ordering of dopants and oxygen vacancies is studied for Gd-doped ceria (x(Gd) <= 0.25) by means of a combined density functional theory (DFT) and cluster expansion approach, where the cluster interactions derived from DFT calculations are further used in Monte Carlo simulations. The methodology is meticulously tested and the stability of the obtained solutions with respect to the volume change, applied exchange-correlation approximation and other modelling parameters is carefully analysed. We study Gd and vacancy ordering in the case of thermodynamic equilibrium and vacancy ordering for quenched Gd configurations. We find that at the thermodynamic equilibrium there exists a transition temperature (T-C) below which phase separation into C-type Gd2O3 and pure CeO2 occurs. The phase separation is observed in the whole studied concentration range and the transition temperature increases with concentration from ca. 600 (x(Gd) = 0.03) to 1000 K (x(Gd) = 0.25). Above T-C the distribution of Gd is random, oxygen vacancies tend to cluster in the coordination shells along < 1, 1/2, 0 > and < 1, 1, 1 >, and the nearest neighbour position is preferred for Gd-vacancy. In the quenched Gd case, where Gd atoms are immobilised below 1500 K, the vacancy ordering is significantly frustrated. In fact, we observe an oxygen freezing transition below temperature T-F approximate to T-C - 350 K, which is close to temperatures at which a change in the conductivity slope is observed experimentally.
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| 8. |
- Zguns, Pjotrs A., et al.
(författare)
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Phase diagram and oxygen-vacancy ordering in the CeO2-Gd2O3 system : a theoretical study
- 2018
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 20:17, s. 11805-11818
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Tidskriftsartikel (refereegranskat)abstract
- We present the phase diagram of Ce1-xGdxO2-x/2 (CGO), calculated by means of a combined Density Functional Theory (DFT), cluster expansion and lattice Monte Carlo approach. We show that this methodology gives reliable results for the whole range of concentrations (x x(Gd) <= 1). In the thermodynamic equilibrium, we observe two transitions: the onset of oxygen-vacancy (O-Va) ordering at ca. 1200-3300 K for concentrations x(Gd) = 0.3-1, and a phase separation into CeO2 and C-type Gd2O3 occurring below ca. 1000 K for all concentrations. We also model 'quenched' systems, with cations immobile below 1500 K, and observe that the presence of random-like cation configurations does not prevent C-type vacancy ordering. The obtained transition temperatures for Va ordering agree rather well with existing experimental data. We analyse the effect of vacancy ordering and composition on the lattice parameters and relaxation pattern of cations.
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| 9. |
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| 10. |
- Žguns, Pjotrs
(författare)
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First Principles Modelling of Clean Energy Materials
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis presents the density functional theory study on clean energy materials relevant for catalysis applications, and for solid oxide fuel cells.In the first part of the thesis the metal supported ultrathin films, namely ScN/Mo, MgO/Mo and NaF/Mo are considered, and the Cu atom adsorption and charging on them is explored.The comparative study of these different films allows us to provide recommendations regarding the choice of materials, in order to promote adatom charging. The modulation of the adatom charge, by changing the material of the film, also paves the way for the design of novel catalysts. Moreover, the detailed investigation of the Cu/NaF/Mo caseshows a correlation between the charge redistribution upon the adsorption and the anharmonicity of the accompanying distortion. Overall, the research commands a fresh view on the adatom charging mechanism.In the second part of the thesis the gadolinium doped ceria, used asoxide electrolyte in solid oxide fuel cells, is studied. The employment of the cluster expansion method together with the density functional theory calculations provides the description of the configurational energy spectrum of dopants and oxygen vacancies in terms of effective pair and three site interactions. The chosen method allows one to predict the energy of anarbitrary configuration. Moreover, the effect of volume change on the strength of interactions is investigated, which is relevant for the modelling ofoxide electrolytes at operating temperatures of solid oxide fuel cells,i.e. when volume expansion is notable.
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