Universität Osnabrück, Graduiertenkolleg Mikrostruktur oxidischer

Weitere Magazine

Empfehlungen

Info

24 UNIVERSITÄT OSNABRÜCK, FACHBEREICH PHYSIK • • • • • • 9 M. Demeter, V. Cherkashenko, A. R. Postnikov, V. R. Galakhov, E. Z. Kurmaev and M. Neumann, Electronic structure of the mixed- valence system V2-xMoxO5, accepted for publication in Surf. Sci. 10 V. Tsurkan, M. Demeter, S. Plogmann and M. Neumann, X-Ray Photoelectron Spectroscopic Studies of Some Ternary Magnetic Chalcogenide Semiconductors, accepted for publication in Jpn. J. of Appl. Phys. 11 M. Demeter, V. Galakhov, N. I. Labachevskaya, E. Z. Kurmaev and M. Neumann, Electronic study of the La-Mn-O system by XPS and XES, accepted for publication in Acta Polonica A 12 S. G. Chiuzbaian, M. Coldea, M. Neumann, D. Todoran, M. Demeter, R. Tetean and V. Pop, Magnetic and electronic properties of GdNi5-xAlx intermetallic compounds, , accepted for publication in Acta Polonica A 13 M. Coldea, M. Neumann, M. Demeter, V. Pop, Mixed-valence state of Ce ions in CeNi2Al3, submitted to J. of Alloys and Compounds 14 V. Galakhov, M. Demeter, S. Bartkowski, M. Neumann, N. A. Ovechkina, E. Z. Kurmaev and N. I. Labachevskaya, Mn 3s exchange splitting in mixed-valence manganites, to be submitted Besuchte Lehrveranstaltungen Ringvorlesung des Graduiertenkollegs Seminar des Kollegs Kolloquium des SFB 225 Vorlesungen: Prof. Dr. M. Coldea, Magnetism and magnetic properties of materials, 1999 Prof. Dr. E. A. Kotomin, Introduction into theory of defects in oxide solids, 98/99 Konferenzbesuche, Forschungsaufenthalte, auswärtige Vorträge Poster presentations at: • International Conference on Solid State Crystals: Marerials Science and applications, Zakopane (Poland), 12-16 October 1998 (Investigation of the mixed-valent system V2-xMoxO5 by XPS) • Deutsche Physikalische Gesellschaft- Früjahrstagung, Münster, 22-26 March, 1999 (Investigation of the La- Mn-O system by XPS and XES) • Deutsche Physikalische Gesellschaft- Früjahrstagung, Regensburg, 27-31 March, 2000 (X-ray photoelectron spectroscopic study of ternary magnetic chalcogenide semiconductors) • International Conference “Condensed Matter Physics", Ustron-Jaszowiec (Poland), 17-20 May, 2000 (Electronic structure of the doped La-Mn-O perovskites) • 18 th European conference on Surface Science (ECOSS-18), Vienna (Austria), 21-24 September, 1999 (Mixed-valence vanadium oxides studied by XPS) • 19 th European conference on Surface Science (ECOSS-19), Madrid (Spain), 5-8 September, 2000 (Electronic structure of the mixed-valent system V2-xMoxO5) Oral presentations at: • Workshop of the Graduiertenkolleg on Electronic Structure of Oxides and Intermetallic compounds, Osnabrück, 24-25 November, 1998 (title: XPS study of some vanadium oxides) • International workshop on Microstructure of Oxide Material , Osnabrück, June 13-15, 2000 (title: X-ray photoelectron spectroscopic study of ternary magnetic chalcogenides) Others: • III Training Course in the Physics of Correlated Electron Systems and High TC Superconductors, IIASS, Vietri sul Mare (Italy), 14-25 September, 1998 • Low-temperature XPS measurements at the Silesian University of Katowice, Poland, 15-30 Mai 1999 • Visit at the Institute of Metal Physics, Academy of Sciences, Ekaterinburg, Russia + poster presentation at the Russian-German Seminar on X-ray photoelectron spectroscopy, Ekaterinburg, 11-20 September 1999
GRADUIERTENKOLLEG MIKROSTRUKTUR OXIDISCHER KRISTALLE 25 Computer simulations of defects in KNbO3 and KTaO3 Beginn des Projekts: 01.02.1997 Ende des Projekts: 31.03.1998 Dr. Roberts Eglitis (Postdoktorand) Betreuer: Prof. Dr. G. Borstel Zusammenfassung Semi-empirical Hartree-Fock calculations using the intermediate neglect of differential overlapp (INDO) method, for self-ordered cubic symmetry clusters of seven Nb ions in KTaO3 are performed with the aim of verifying the cluster model [1] of second component-induced phase transitions in ferroelectric perovskite matrices. It is shown that such a seven-particle cluster in KTaO3:Nb has two types of quasidegenerate states of different nature. Namely, the state with the equilibrium full-symmetric dilatation and off-center displacements in [111]-directions of the central Nb-ion in the cluster, and the state with the equilibrium full-symmetric compression without any off-center effect. The consequencies of such cluster structures on the multi-well potential are discussed. In particular, an avalanche-like behaviour of the temperature dependence of the ferroelectric order parameter is obtained. The results of semi-empirical INDO calculations of a seven-impurities cluster of Nb-ions in KTaO3 confirm the key assumptions of the cluster model of induced phase transitions. The LMTO and INDO methods are used for a supercell study of F centers in cubic and orthorhombix ferroelectric KNbO3 crystals. Two electrons are found to be considerably delocalized even in the ground state of the defect. The absorption energies are calculated by means of the INDO method using the ∆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. 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. We used INDO method for large unit cell calculations of hole polarons bound to a cation vacancy in partly covalent perovskite KNbO3. In KNbO3 we predict the existence of one-site and two site (molecular) polarons with close absorption energies ∼ 1 eV. Ergebnisse The exploration of the Nb-clusters in KTaO3 was performed using the periodic LUC method, as it is implemented in the updated CLUSTERD computer code [I,II,2,3]. In order to calculate the Nb clusters in KTaO3, we replaced in our 4×4×4 times extended unit cell, containing 320 atoms, seven Ta atoms by seven Nb atoms. After this, in order to find the energy minima of Nb clusters in KTaO3, we allowed to six Nb atoms to relax towards the central Nb atom. The results of our calculations show that for six Nb atoms shifted symmetrically towards the central Nb atom by 0.187 Å, the total energy per LUC is lowered by 0.088 eV. Moreover, the outward uniform displacement of six Nb atoms from the central Nb by 0.073 Å is also favorable and lowers the energy by approximately 0.03 eV. In the case, when 6 Nb atoms are shifted outwards from the central Nb atom, the central Nb-atom moves off-center from central position in the [111] direction by 0.27 Å, further lowering the total energy of system by 0.09 eV. These configurations correspond to a ground-state vibronic multiplet. The Nb atom reveals also an instability in the [100] direction. The shift of the central Nb ion along the [100] direction by 0.192 Å lowers the cluster energy additionally by 0.056 eV in the same case when six Nb atoms are shifted outwards from the central Nb atom. The total cluster-structure induced energy lowering in the ground state, which corresponds to the situation when six Nb atoms are symmetrically shifted in the direction from the center of the Nb cluster, and the central Nb atom is off-center in the [111] direction is 0.12 eV [III]. According
Seite 1 und 2: Universität Osnabrück Graduierten
Seite 3 und 4: GRADUIERTENKOLLEG MIKROSTRUKTUR OXI
Seite 23: GRADUIERTENKOLLEG MIKROSTRUKTUR OXI
Seite 49 und 50: ODMR Signal GRADUIERTENKOLLEG MIKRO
Seite 75 und 76:
GRADUIERTENKOLLEG MIKROSTRUKTUR OXI
Seite 77 und 78:
GRADUIERTENKOLLEG MIKROSTRUKTUR OXI
Alle anzeigen

Universität Osnabrück, Graduiertenkolleg Mikrostruktur oxidischer

Sie wollen auch ein ePaper? Erhöhen Sie die Reichweite Ihrer Titel.

Template löschen?

Als Template speichern?