Peter C. Schmidt

Iron-oxygen vacancy defect centers in PbTiO3 : Newman superposition model analysis and density functional calculations

The iron(III) center in ferroelectric PbTiO3 together with an oxygen vacancy forms a charged defect associate, oriented along the crystallographic c-axis. Its microscopic structure has been analyzed in detail comparing results from a semi-empirical Newman superposition model analysis based on finestructure data and from calculations using density functional theory. Both methods give evidence for a substitution of Fe3+ for Ti4+ as an acceptor center. The position of the iron ion in the ferroelectric phase is found to be similar to the B-site in the paraelectric phase. Partial charge compensation is locally provided by a directly coordinated oxygen vacancy. Using high-resolution synchrotron powder diffraction, it was verified that lead titanate remains tetragonal down to 12 K, exhibiting a c/a-ratio of 1.0721.

A chemically driven insulator-metal transition in non-stoichiometric and amorphous gallium oxide.

Insulator-metal transitions are well known in transition-metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here, we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO(1.2) at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga(2)O(3) within the metastable oxide matrix-in chemical terms by a disproportionation. This novel mechanism--an insulator-metal transition driven by a heterogeneous solid-state reaction--opens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.

Changes in the crystal and electronic structure of LiCoO2 and LiNiO2 upon Li intercalation and de-intercalation

Li(x)CoO(2) and Li(x)NiO(2) (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of Li(x)CoO(2) and Li(x)NiO(2) as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Li(x)(Co,Ni)O(2) (0 < x < or = 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.

Theoretical and experimental determination of the electronic structure of V2O5, reduced V2O5-x and sodium intercalated NaV2O5

In this work the electronic structure of V(2)O(5), reduced V(2)O(5-x) (V(16)O(39)) and sodium intercalated NaV(2)O(5) has been studied by both theoretical and experimental methods. Theoretical band structure calculations have been performed using density functional methods (DFT). We have investigated the electron density distribution of the valence states, the total density of states (total DOS) and the partial valence band density of states (PVBDOS). Experimentally, amorphous V(2)O(5) thin films have been prepared by physical vapour deposition (PVD) on freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates at room temperature with an initial oxygen understoichiometry of about 4%, resulting in a net stoichiometry of V(2)O(4.8). These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV(2)O(5)) at room temperature. Resonant V3p-V3d photoelectron spectroscopy (ResPES) experiments have been performed to determine the PVBDOS focusing on the calculation of occupation numbers and the determination of effective oxidation state, reflecting ionicity and covalency of the V-O bonds. Using X-ray absorption near edge spectra (XANES) an attempt is made to visualize the changes in the unoccupied DOS due to sodium intercalation. For comparison measurements on nearly stoichiometric V(2)O(5) single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V(2)O(5) single crystals and the NaV(2)O(5) results are in good agreement with the calculated values. The ResPES results for V(2)O(4.8) agree in principle with the calculations, but the trends in the change of the ionicity differ between experiment and theory. Experimentally we find partly occupied V 3d states above the oxygen 2p-like states and a band gap between these and the unoccupied states. In theory one finds this occupation scheme assuming oxygen vacancies in V(2)O(5) and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV(2)O(5.).

Investigations of Nuclear Quadrupole Interaction in BaMgAl10O17:Eu2+

Local environments of divalent europium ions in β-lattice of barium magnesium aluminate, BaMgAl10O17 (BAM) have been investigated using a recently developed ab initio band structure method. This method is a variant of the full potential LMTO method. The reliability of this method for calculating electric field gradient (EFG) was tested in the case of aluminum oxide in corundum structure. The calculated EFGs at both cation and anion sites compare satisfactorily with those from other ab initio methods and from recent measurements. The nuclear quadrupole coupling constants for Eu2+ calculated by this method at various possible sites in BAM, when compared to recently measured 151Eu Mössbauer spectroscopy results, suggest three locations for Eu2+: a Beevers–Ross site, a mid-oxygen site and an anti-Beevers–Ross site. Energetically, the anti-Beevers–Ross site appears to be more stable than the other two sites.

A First-Principles Investigation of the Electronic Structure of Trivalent Rare Earth Ions in Gallium Nitride

Using first-principles electronic structure methods, we have investigated the electronic structure and associated properties of Pr3+, Nd3+, Eu3+, Tb3+, Dy3+ and Er3+ substituting for Ga3+ in GaN. It is found that these rare earth ions (RE3+) are stable at the Ga site in tetrahedral coordination with nitrogen atoms. The nitrogen ligands move away from the RE3+ site to allow for the large ionic radii of the RE3+ ions. The equilibrium bond lengths of RE–N are found to vary between 2.15 to 2.30Å in good agreement with structural data available for Eu3+, Tb3+ and Er3+. Using the calculated energy bands, excitation energies for the 4f→5d transition have also been predicted. Most of these energies are larger than the band gap of GaN and cannot be exploited in transferring host excitation energies to the RE3+ ions.

On the structural and magnetic properties of R2Fe17-x(A, T)x (R = rare earth; a = Al, Si, Ga; T = transition metal) compounds

R 2 Fe 17 (R = rare earth) intermetallic compounds constitute one of the most important classes of materials identified as high-energy permanent magnet materials. They crystallize either in the rhombohedral Th 2 Zn 17 structure (for light R) or in the hexagonal Th 2 Ni 17 structure (for heavy R). In this article, we discuss the variations in the lattice parameters (unit cell volume), site occupancies and Curie temperature when non-transition and transition metals are substituted for Fe in R 2 Fe 17 compounds.

Optimum laser parameters for 1D radiation pressure acceleration

Laser ion acceleration (Wilks et al ., 2001; Passoni et al ., 2010) has become an interesting field of research in the past years. Several experiments, such as LIGHT (Schollmeier et al ., 2008; Bagnoud et al ., 2010; Busold et al ., 2013; 2014 a ; 2014 b ) are performed worldwide. High intense, pulsed laser beams are used to generate and accelerate a plasma. For higher laser intensities (>10 21 W cm −1 ), simulations (Esirkepov et al ., 2004; Macchi et al ., 2005; 2009; 2010; Robinson et al ., 2008; Rykovanov et al ., 2008; Henig et al ., 2009; Schlegel et al ., 2009; Shoucri et al ., 2011; 2013; 2014; Kar et al ., 2012; Korzhimanov et al ., 2012; Shoucri, 2012) have revealed a new acceleration mechanism: The Radiation Pressure Acceleration. The entire foil target is accelerated by the radiation pressure of the laser pulse. Ideally, a sharp peak spectrum is generated, with energies up to GeV and nearly solid body density. This work faces on a detailed analysis of the acceleration mechanism in order to develop the optimum laser- and target parameters for the process. The analysis is supported by one-dimensional PIC simulations, using the commercial code VSim

Theoretical Investigation of Intercalated Water Molecules and Hydroxyl Groups in BAM ( BaMgAl10O17 : Eu2 + ) Phosphor and Associated Degradation Processes

Intercalation of water molecules into the p-alumina lattice of barium magnesium aluminate (BaMgAl 10 O 17 ) has been recently reported to be responsible for degradation of this phosphor. The associated degradation process is characterized by a green shift of the emission spectrum accompanied by oxidation of the activator ions. However, the structure and location of intercalated water molecules, and the degradation mechanism are not understood. In this report, both the location and geometry of water molecules are investigated using an atomistic simulation method with empirical interatomic potentials and a quantum mechanical method of structure optimization using full periodicity of the lattice. The oxidation of activator ions is modeled based on dissociation of water molecules to hydroxyl groups in this lattice. It is shown that the dissociation to hydroxyl groups is energetically favorable, and that the protons released in the process could be responsible for oxidation of europium ions.

Dependence of Phase Composition and Luminescence of Sr6BP5O20 on Eu Concentration

We have investigated how secondary phases appear and disappear upon increasing europium concentration from 1.5 to 16.2 atom % in standard strontium borophosphate phosphate phosphor, Sr 6 BP 5 O 20 , using X-ray diffraction and photoluminescence and photoluminescence excitation measurements both at room temperature and at 80 K. In standard phosphor, a minor phase of strontium phosphate, Sr 3 (PO 4 ) 2 , was detected besides the main component Sr 6 BP 5 O 20 . With increasing Eu concentration, the phase Sr 3 (PO 4 ) 2 disappears and new phases of Sr 3 Eu(PO 4 ) 3 , SrBPO 5 , and Sr 2 P 2 O 7 are formed. Sr 3 Eu(PO 4 ) 3 is observed to be the major phase in 16.2 atom % samples. It contains only trivalent europium. An attempt has also been made to understand the origin of these secondary phases and their significance for the performance of this luminescent material. This study shows how X-ray diffraction technique and photoluminescence spectroscopy can be used in a complementary manner to explore the phase composition of the materials.