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«ANNUAL REPORT Riga 2012 Annual Report 2011, Institute of Solid State Physics, University of Latvia. Editor: A.Krumins. Composed matter: A.Muratova. ...»

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AND MIGRATION IN BA1-XSrXCo1-YFeYO3 PEROVSKITES Yu. Mastrikov, E.A. Kotomin, R. Merkle, J. Maier (Max Planck Institute for Solid State Research, Stuttgart, Germany) M. M.Kuklja (Materials Science and Eng.Dept., University of Maryland, College Park, USA) Based on first principles DFT calculations, we analyzed oxygen vacancy formation and migration energies as a function of chemical composition in complex multicomponent (Ba,Sr)(Co,Fe)O3 perovskites which are candidate materials for SOFC cathodes and permeation membranes. The atomic relaxation, electronic charge redistribution and energies of the transition states of oxygen migration are compared for several perovskites to elucidate the atomistic reason for the exceptionally low migration barrier in Ba0.5Sr0.5Co0.8Fe0.2O3 that was previously determined experimentally. The critical comparison of Ba1-xSrxCo1-yFeyO3 perovskites with different cation compositions and arrangements shows that in addition to the geometric constraints the electronic structure plays a considerable role for the height of the oxygen migration barrier in these materials. These findings help to understand the fast oxygen permeation and exchange properties of Ba0.5Sr0.5Co0.8Fe0.2O3 as well as some of its limitations

PHONON CALCULATIONS IN CUBIC AND TETRAGONAL PHASES OF SrTiO3:

A COMPARATIVE LCAO AND PLANE WAVE STUDY

D. Gryaznov, E.A. Kotomin, R.A. Evarestov (St. Petersburg State University, Peterhof, Russia) E. Blokhin, J. Maier (Max-Planck Institute for Solid State Research, Stuttgart, Germany) The atomic, electronic structure and phonon frequencies have been calculated in a cubic and low-temperature tetragonal SrTiO3 phases at the ab initio level. We demonstrated that the use of hybrid exchange-correlation PBE0 functional gives the best agreement with experimental data. The results for the standard Generalized Gradient Approximation (PBE) and hybrid PBE0 functionals are compared for the two types of approaches: a linear combination of atomic orbitals (CRYSTAL09 computer code) and plane waves (VASP 5.2 code). Relation between cubic and tetragonal phases and the relevant antiferrodistortive phase transition is discussed in terms of group theory and illustrated with analysis of calculated soft mode frequencies at the Γ- and R-points in the Brillouin zone. Based on phonon calculations, the temperature dependence of the heat capacity is in a good agreement with experiment. This approach is promising for defect modeling in many advanced materials under high operational temperatures.

FIRST-PRINCIPLES CALCULATIONS OF THE ATOMIC AND ELECTRONIC

STRUCTURE OF PEROVSKITE SURFACES

R.I. Eglitis The ab initio calculations of polar CaTiO3 (111) surface were performed. Surface relaxation, rumpling, energetics, optical band gaps, and charge distribution are obtained using the computer code CRYSTAL and a hybrid exchange-correlation functionals. Using a hybrid B3LYP approach, the surface relaxation for the two possible Ti and CaO3 (111) surface terminations are calculated. For both Ti and CaO3-terminated CaTiO3 (111) surfaces upper layer atoms relax inwards, while the second layer atoms, with the sole exception of CaO3terminated surface Ca atom, relax outwards. The alculated surface relaxation energy for Titerminated surface is more than five times larger than the surface relaxation energy for CaO3terminated surface. The surface energy for Ti-terminated surface (4.18 eV/cell) is smaller, than the surface energy for CaO3-termination (5.86 eV/cell).

We calculated also properties of the SrZrO3 (001) surfaceBoth SrO and ZrO2 terminations were considered. On the (001) surfaces all upper and third layer atoms relax inward, while outward relaxations of all atoms in the second layer are found with the sole exception of SrOterminated SrZrO3 (001) surface second layer O atom. Calculated surface rumpling for the SrO-terminated SrZrO3 (001) surface (6.77 % of the lattice constant) is by a factor of ten larger than the surface rumpling for the ZrO2-terminated surface (0.72 % of ao). A considerable increase in the Zr-O chemical bond covalency near the SrZrO3(001) surface as compared to the bulk is predicted.

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The F center (an electron trapped by a fluorine vacancy) and R center (a defect composed of three F centers) in BaF2 crystal have been studied using density functional theory (DFT) with hybrid exchange-correlation DFT-B3PW functional. Our calculations show that the F-center transfer barrier is equal to 1.83 eV. The association energy calculations on R centers indicate energy gain with respect to three isolated F centers. During F-center aggregation, a considerable covalency arises between two neighbor fluorine vacancies with trapped electrons. Three incompletely paired electrons trapped in the R center have an up-down-up spin arrangements and induce three defect levels in the gaps between valence bands (VB) and conduction bands (CB) for both α- and β-spin polarized band structures, respectively. More defect bands lead to more complex electron transitions, which were classified into two F- and four M-like transitions. The DOS calculations clearly reveal the components of defect bands.

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Over the past few decades an enhanced interest takes place to the studies of silver-containing fast ionic conductors. These crystals are, first of all, of interest due to phenomena of the reversible transformation of luminescence centers structure as well as mobile silver ions concentration changes within the local irradiated region of the crystals.





This work completes our thorough investigation of Ag2CdI4 solid electrolyte. In addition to previous results of electrical, optical and thermal properties of this model compound and following recent report on its microstructure studied by SEM, impedance spectroscopy and fractal dimension analysis as well as infrared spectra and phonon density of states calculation, herewith we focused on the band energy structure calculations of the Ag2CdI4 single crystals and determination of partial densities of states in the valence band.

Energy band dispersion calculations have been performed for Ag2CdI4 superionic within a framework of local density approximation (Perdew–Zunger parameterization) exploiting the first-principles CASTEP computer code. The ab-initio electronic structure simulations were performed for both types of ε-Ag2CdI4 crystalline structures. Principal optical functions as well as the density of electronic states in the spectral range of inter-band optical transitions (2.5 eV–20 eV) were determined. Theoretically calculated absorption coefficients derived from the obtained band structure are compared with appropriate experimental data.

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The kinetics of pattern formation and phase separation in a closed system of two types of oppositely charged molecules with competing short- and long-range interactions on surfaces/interfaces was studied combining three methods: a microscopic formalism of the joint correlation functions, Reverse Monte Carlo, and non-equilibrium charge screening factors. The molecular ordering occurs on the background of the Ostwald ripening and thus is strongly non-equilibrium. It is demonstrated how initial random distribution of molecules is changed for loose similar-molecule aggregates, with further reorganization into dense macroscopic domains of oppositely charged molecules (Fig. 14). Pattern formation process is characterized by the correlation length which monotonically increases in time.

The kinetics of pattern formation was also studied for reversible A+B→0 reaction of mobile oppositely charged molecules at the interface. Using formalism of joint correlation functions, non-equilibrium charge screening and reverse Monte-Carlo methods, it is shown that labyrinth-like percolation structure induced by (even moderate-rate) reaction is principally non-steady-state and is associated with permanently growing segregation of similar reactants and aggregation of similar reactants into domains. A role of short-range and long-range reactant interactions is discussed.

Fig. 14. Fragments of the characteristic snapshots obtained using the reverse Monte Carlo for the correlation functions In collaboration with Northwestern University, Evanston, USA, the effects of non-equilibrium charge screening in mixtures of oppositely charged interacting molecules on surfaces are analyzed in a closed system. The dynamics of charge screening and the strong deviation from the standard Debye-Hückel theory are demonstrated via a new formalism based on computing radial distribution functions suited for analyzing both short-range and long-range special ordering effects. At long distances, the inhomogeneous molecule distribution is limited by diffusion, whereas at short distances (of the order of several coordination spheres) by a balance of short-range (Lennard-Jones) and long-range (Coulomb) interactions (Fig. 15). The non-equilibrium charge screening effects in transient pattern formation are further quantified.

It is demonstrated that use of screened potentials, in the spirit of the Debye-Hückel theory, leads to qualitatively incorrect results.

Fig. 15. Ionic binary systems with Lennard-Jones and Coulomb interactions at low temperatures for intermediate density: (a) the joint correlation functions; (b) the partial structure factors; (c) and (d) the non-equilibrium screening factors.

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Larger scale plasma instabilities not leading to an immediate termination of a discharge often result in periodic nonlinear perturbations. A simplest possible model is suggested for description of the system with drive and relaxation processes with different time scales. The model is based on two equations: the first being responsible for the relaxation dynamics and the second one for the drive (Fig. 16). The model can be generalized to describe the pellet injection.

–  –  –

UNDERSTANDING COMPLEX MAGNETOHYDRODYNAMIC ACTIVITIES WITH

A RELAXATION IN THE HT-7 TOKAMAK

Erzhong Li1, Liqun Hu1, V Igochine2, O Dumbrajs3 and Kaiyun Chen1 Institute of Plasma Physics, Chinese Academy of Science, Hefei, China MPI f¨ur Plasmaphysik, Euratom-Association, D-85748 Garching, Germany Institute of Solid State Physics, Association Euratom-University of Latvia, Riga, Latvia A new relaxation instability with complex magnetohydrodynamics (MHD) activities is found in the HT-7 tokamak operational region, which manifests itself in bursts of hydrogen alpharay radiations, electron cyclotron emission and soft x-ray (SX) radiations on outer channels, as well as complex MHD perturbations, but without hard disruptions. It is found that a stochastic annular belt resulted just before the relaxation due to the m/n = 5/3 island overlapping with m/n = 2/1 and m/n = 3/2 islands (Fig. 17).

–  –  –

O Dumbrajs Theory describing the effect of reflections on operation of gyrotrons with radial output is applied the ITER 170 GHz 2 MW coaxial cavity gyrotron, which is under development, and to the 170 GHz 1 MW cylindrical cavity gyrotron as a fall back solution.

–  –  –

SCI

1. R.A. Evarestov, Yu.F. Zhukovskii, A.V. Bandura, S. Piskunov, and M.V. Losev, Symmetry and models of double-wall BN and TiO2 nanotubes with hexagonal morphology. – J. Phys.

Chem. C, 2011, 115, p. 14067–14076.

2. S. Piskunov, T. Jacob, and E. Spohr, Oxygen adsorption at La1-xSrxMnO3(001) surfaces:

Predictions from first principles. - Phys. Rev. B, 2011, 83, 073402 (p. 1-4).

3. R.A. Evarestov, E. Blokhin, D. Gryaznov, E.A. Kotomin, and J. Maier, Phonon calculations in cubic and tetragonal phases of SrTiO3: A comparative LCAO and plane-wave study. - Phys.

Rev. B, 2011, 83, 134108 (p. 1-9).

4. V.N. Kuzovkov, E.A. Kotomin, and G. Zvejnieks, Pattern formation kinetics for charged molecules on surfaces: microscopic correlation function analysis. - J. Phys. Chem. B, 2011, 115, p. 14626–14633.

5. E.A. Kotomin, V. Aleksandrov, D. Gryaznov, R.A. Evarestov, and J. Maier, Confinement effects for ionic carriers in SrTiO3 ultrathin films: first-principles calculations of oxygen vacancies. - Phys. Chem. Chem. Phys., 2011, 13, p. 923-926.

6. V.N. Kuzovkov, E.A. Kotomin, and G. Zvejnieks, Atomistic theory of mesoscopic pattern formation induced by bimolecular surface reactions between oppositely charged molecules. J. Chem. Phys., 2011, 135, 224503 (p. 1-5).

7. V.N. Kuzovkov, E.A. Kotomin, and M. Olvera de la Cruz, The non-equilibrium charge screening effects in diffusion-driven systems with pattern formation. - J. Chem. Phys., 2011, 135, 034702 (p. 1-6).

8. E.A. Kotomin, Yu.A. Mastrikov, M.M. Kuklja, R. Merkle, A. Roytburd, and J. Maier, First principles calculations of oxygen vacancy formation and migration in mixed conducting Ba0.5Sr0.5Co1-yFeyO3-δ perovskites - Solid State Ionics (2.496), 2011, 188, p. 1-5.

9. S. Velgosh, B. Andriyevsky, I. Karbovnyk, I. Bolesta, O. Bovgyra, W. Ciepluch-Trojanek, I.V. Kityk, and A.I. Popov, First-principles simulations of the electronic density of states for superionic Ag2CdI4 crystals. - Solid State Ionics, 2011, 188, p. 31-35.

10. H. Shi, R. Jia, and R.I. Eglitis, First-principles simulations on the aggregation of F centers in BaF2: R center. - Solid State Ionics, 2011, 187, p. 1-7.

11. E. Li, L. Hu, V. Igochine, O. Dumbrajs, and K. Chen, Understanding complex magnetohydrodynamic activities associated with a relaxation in the HT-7 tokamak. - Plasma Phys. Control. Fusion, 2011, 53, 085019 (p. 1-13).

12. D. Constantinescu, O. Dumbrajs, V. Igochine, K. Lackner, R. Meyer-Spasche, H. Zohm, and ASDEX Upgrade Team, A low-dimensional model system for quasi-periodic plasma perturbations. - Phys. Plasmas (2.320), 2011, 18, 062307 (p. 1-7).

13. V. Pankratov, A.I. Popov, L. Shirmane, A. Kotlov, and C. Feldmann, LaPO4:Ce,Tb and YVO4:Eu nanophosphors: Luminescence studies in the vacuum ultraviolet spectral range. - J.

Appl. Phys., 2011, 110, 053522 (p. 1-7).

14. D. Bocharov, D. Gryaznov, Yu.F. Zhukovskii, and E.A. Kotomin, DFT calculations of point defects on UN(001) surface. - Surf. Sci., 2011, 605, p. 396-400.



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