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E. A. Kotomin

Publications and source records attributed to E. A. Kotomin.

10 recordsLinked to original sources
arXiv2012

Ab initio simulations of oxygen interaction with surfaces and interfaces in uranium mononitride

The results of DFT supercell calculations of oxygen behavior upon the UN (001) and (110) surfaces as well as at the tilt grain boundary are presented. Oxygen adsorption, migration, incorporation into the surface N vacancies on (001) and (110) surfaces have been modeled using 2D slabs of different thicknesses and supercell sizes. The temperature dependences of the N vacancy formation energies and oxygen incorporation energies are calculated. We demonstrate that O atoms easily penetrate into UN surfaces and grain boundaries containing N vacancies, due to negative incorporation energies and a small energy barrier. The Gibbs free energies of N vacancy formation and O atom incorporation therein at the two densely-packed surfaces and tilt grain boundaries are compared. It has been also shown that the adsorbed oxygen atoms are highly mobile which, combined with easy incorporation into surface N vacancies, explains efficient (but unwanted) oxidation of UN surfaces. The atomistic mechanism of UN oxidation via possible formation of oxynitrides is discussed.

cond-mat.mtrl-sci↗
arXiv2010

Helium Behavior in Oxide Nuclear Fuels: First Principles Modeling

UO2 and (U,Pu)O2 solid solutions (the so-called MOX) nowadays are used as commercial nuclear fuels in many countries. One of the safety issues during the storage of these fuels is related to their self-irradiation that produces and accumulates point defects and helium therein. We present density functional theory (DFT) calculations for UO2, PuO2 and MOX containing He atoms in octahedral interstitial positions. In particular, we calculated basic MOX properties and He incorporation energies as functions of Pu concentration within the spin-polarized, generalized gradient approximation (GGA) DFT calculations. We also included the on-site electron correlation corrections using the Hubbard model (in the framework of the so-called DFT+U approach). We found that PuO2 remains semiconducting with He in the octahedral position while UO2 requires a specific lattice distortion. Both materials reveal a positive energy for He incorporation, which, therefore, is an exothermic process. The He incorporation energy increases with the Pu concentration in the MOX fuel.

cond-mat.mtrl-sci↗
arXiv2009

Void superlattice formation as self-organization phenomemon. 1. Scaling estimates

The literature on void superlattice formation observed under irradation of metals and insulators is analyzed with a special attention to the self-organization; a general scenario of this process is discussed. A simple relation for the superlattice parameter as a function of the dose rate and temperature is suggested, in a good agreement with existing experimental data. Special attention is paid to analysis of the halogen gas void formation in an electron-irradiated CaF2

cond-mat.mtrl-sci↗
arXiv2004

Ab initio calculations of the atomic and electronic structures for ABO3 perovskite (001) surfaces

We present the results of first-principles calculations on two possible terminations of the (001) surfaces of SrTiO3 (STO), BaTiO3 (BTO), and PbTiO3 (PTO) perovskite crystals. Atomic structure and the electronic configurations were calculated for different 2D slabs, both stoichiometric and non-stoichiometric, using hybrid (B3PW) exchange-correlation technique and reoptimized basis sets of atomic (Gaussian) orbitals. Results are compared with previous calculations and available experimental data. The electronic density distribution near the surface and covalency effects are discussed in details for all three perovskites.

cond-mat.mtrl-sci↗
arXiv2001

Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and α-Al_2O_3 (corundum). A continuum theory at arbitrary electron-phonon coupling is used for calculation of the energies of thermal dissociation, photoionization (optically induced release of an electron (hole) from the ground self-consistent state), as well as optical absorption to the non-relaxed excited states. Unlike the case of free strong-coupling polarons, where the ratio κof the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Froehlich coupling constant αand the screened Coulomb interaction strength β. Reasonable variation of these two parameters has demonstrated that the magnitude of κremains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation energy for the ground self-consistent state and the energy of the optically induced charge transfer process (hops of a hole between O^{2-} ions) have been calculated using the quantum-chemical method INDO. Results obtained within the two approaches for hole O$^-$ polarons bound by the cation vacancies (V^-) in MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to experimental data and to each other. We discuss a surprising closeness of the results obtained on the basis of independent models and their agreement with experiment.

cond-mat.str-el↗
arXiv1999

First-principles and semi-empirical calculations for bound hole polarons in KNbO3

The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Intermediate Neglect of the Differential Overlap (INDO) based on the Hartree-Fock formalism are combined for the study of the hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of KNbO3 perovskite crystals. The 40-atom and 320-atom supercells were used, respectively. We predict existence of both, one-site and two-site (molecular) polarons with close optical absorption energies (0.9 eV and 0.95 eV). The relevant experimental data are discussed.

cond-mat.mtrl-sci↗
arXiv1998

Computer simulations of defects in perovskite KNbO3 crystals

An ab initio LMTO approach and semi-empirical quantum chemical INDO method have been used for supercell calculations of basic point defects - F-type centers and hole polarons bound to cation vacancy - in partly covalent perovskite KNbO3. We predict the existence of both one-site and two-site (molecular) polarons with close absorption energies (~ 1 eV). The relevant experimental data are discussed and interpreted.

cond-mat.mtrl-sci↗
arXiv1998

First-principles and semiempirical Hartree-Fock calculations for F centers in KNbO3 and Li impurities in KTaO3

The LMTO method based on the density-functional theory and the semi-empirical INDO method based on the Hartree--Fock formalism are used for the supercell study of the F centers in cubic and orthorhombic ferroelectric KNbO3 crystals. Two electrons are found to be considerably delocalized even in the ground state of the defect. The absorption energies were calculated by means of the INDO method using the Delta-SCF scheme after a relaxation of atoms surrounding the F center. As an example of another type of point defect in perovskite, an isolated Li impurity in KTaO3 as well as interacting Li pairs are considered in the supercell approach, using the supercells of up to 270 atoms. The off-center Li displacement, reorientational energy barriers and the lattice relaxation around impurities are calculated. The results are compared with those obtained earlier within the shell model, revealing the relaxation pattern somehow different from the shell model estimations.

cond-mat.mtrl-sci↗
arXiv1997

First-principles and semiempirical calculations for F centers in KNbO3

The linear muffin-tin-orbital method combined with density functional theory (local approximation) and the semiempirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the study of the $F$ centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO$_3$ crystals. Calculations for 39-atom supercells show that the two electrons are considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the $F$ center in KNbO$_3$ resembles electron defects in the partially-covalent SiO$_2$ crystal (the so-called $E^{\prime}_1$ center) rather than usual $F$ centers in ionic crystals like MgO and alkali halides. This covalency is confirmed by the analysis of the electronic density distribution. Absorption energies were calculated by means of the INDO method using the $Δ$ self-consistent-field scheme after a relaxation of atoms surrounding the $F$ center. For the orthorhombic phase three absorption bands are calculated to lie at 2.72 eV, 3.04 eV, and 3.11 eV. The first one is close to that observed under electron irradiation. For the cubic phase, stable at high temperatures, above 708 K, only the two bands, at 2.73 eV and 2.97 eV, are expected.

cond-mat.mtrl-sci↗