C. Elsässer

Bulk electronic structure of SrTiO3: Experiment and theory

Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3. Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states. The experimentally determined indirect band gap energy is 3.25 eV, while the direct band gap energy is 3.75 eV. The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV. The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV. The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 3.25 eV up to 15 eV. The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV. The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV. Interband transitions from the O 2s band to the conduction bands appear at 26 eV. A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons.

Microscopic structure and bonding at the rhombohedral twin interface in α-Al2O3

Abstract The local interfacial structure and bonding at the rhombohedral twin interface in α-Al 2 O 3 (corundum) was studied by means of first-principles electronic-structure calculations based on the local density functional theory. Two sets of geometrical interface models were selected, one with a terminating oxygen layer at the interface, the other with a termination by interstitial vacancies of the corundum structure. Optimized interface configurations were obtained by minimization of the total internal energy with respect to relative translation states of the adjoining grains and relaxations of all atomic positions. One vacancy-terminated configuration was found with a very low interface energy, a well-defined relative translation state with screw-rotation symmetry and a highly ordered atomic structure, which minimizes the mutual repulsion between the neighboring like-charged ions. A second metastable configuration with vacancy termination and a higher interface energy was also obtained as well as a metastable oxygen-terminated structure with an interface energy between those of the two vacancy-terminated configurations. The theoretical results for the interfacial structures and translation states are discussed with respect to recent experimental investigations of the twin interface by high-resolution transmission electron microscopy. Furthermore, the calculated interfacial site-projected densities of electron states (PDOS) display significant differences from the bulk-crystal PDOS in the conduction bands.

Valence electron energy loss study of Fe-doped SrTiO3 and a Σ13 boundary: electronic structure and dispersion forces

Valence electron energy loss spectroscopy in a dedicated scanning transmission electron microscope has been used to obtain the interband transition strength of a sigma13 tilt grain boundary in SrTiO3. In a first step the electronic structure of bulk SrTiO3 has been analysed quantitatively by comparing VEELS spectra with vacuum ultraviolet spectra and with ab initio density of states calculations. The electronic structure of a near sigma13 grain boundary and the corresponding dispersion forces were then determined by spatially resolved VEELS. Also the effects of delocalization of the inelastic scattering processes were estimated and compared with results from the literature.

First-principles analysis of cation segregation at grain boundaries in α-Al2O3

Abstract The modifications in atomistic structure, chemical bonding, and energetics induced by substitutional cation impurities isolated in the bulk volume and segregated at grain boundaries of α-Al2O3 were investigated by combining empirical ionic-model and first-principles electronic-structure calculations. The dependency of these modifications on the boundary type, species and concentration of impurities, was studied by selecting the following variety of systems: three twin boundaries (the prismatic Σ3 (10 1 0), the rhombohedral Σ7 (10 1 2), and the pyramidal Σ13 (10 1 4) twins), three impurities X (X=Sc, Y, and La), and two concentrations for the segregant (≈3 and ≈6 atoms/nm2). A partial covalent character is found to be a distinctive feature of the X-O bonds in both bulk and interfacial atomic environments, and to drive the structural distortions of the octahedral XO6 clusters. The energetics of segregation reveals a linear relationship between segregation energy and impurity size. This is interpreted as resulting from a stress field localized at the interface.

Atomistic and electronic structure of Al/MgAl2O4 and Ag/MgAl2O4 interfaces

Abstract For the first time, very precise experimental data on the atomistic structure of a metal/oxide interface were obtained by quantitative high-resolution transmission electron microscopy (HRTEM). They are compared with the results of ab initio density-functional theory (DFT) calculations for the same real interface structure, performed without the need for introducing artificial coherency strains. The model system of this study is the coherent (001)-oriented interface between Al and MgAl2O4 in parallel orientation. By means of quantitative HRTEM we determined the relative translation of the two crystals with picometre precision, and also within this error limit our ab initio calculations correctly predict the experimental structure. The electron density distribution obtained by the calculations indicates a directional bonding between the metal and the oxide beyond the concept of the image charge model. Furthermore, we have carried out ab initio DFT calculations for the (001) interface between Ag and MgAl2O4. Since this interface has the same crystallography as Al/MgAl2O4, comparison of the electron density distribution reveals the net effect of the electron configuration in the metal on the nature of the metal oxide adhesion.

Density-functional modelling of core-hole effects in electron energy-loss near-edge spectra

A series of (MgO)n supercells (n = 1, 4, 8, 16, 32) with three-dimensional periodic boundary conditions is investigated by density-functional band-structure calculations. The influence of supercell size and shape on calculated electron energy-loss near-edge spectra is assessed quantitatively, employing the Z + 1 approximation for the representation of final-state effects. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole and the interaction energy of neighbouring core-hole centres. A sufficient supercell size provided, the Z + 1 approximation yields a highly satisfactory description of excitations from the 1s shell of light elements, such as Mg and O, compared to experimental data. For comparison, pseudopotentials for excited states were generated for Mg, both with a large core (1s, 2s, 2p orbitals) and a small core (1s orbital only) included into the pseudopotential. The corresponding calculations with frozen core holes lead to very good agreement with the results from the Z + 1 calculation for the 1s excitations. The explicit treatment of the subvalence shell (2s, 2p), however, is mandatory for the proper modelling of excitations from orbitals higher than 1s. This indicates that the core polarisability plays an important role in excitations from more extended shells.

Symmetrical tilt grain boundaries in body-centred cubic transition metals: An ab initio local-density-functional study

Abstract Atomic structures and macroscopic translation states for ∑5(310)[001] symmetrical tilt grain boundaries in the body-centred cubic transition metals Nb. Ta, Mo, and W have been calculated in the local-density-functional theory by means of total-energy and force calculations with an ab initio mixed-basis pseudopotential method. For Mo and W translation states of the optimized grain-boundary structures are found with the neighbouring grains being displaced parallel to the [001] tilt axis, yielding grain boundaries without mirror symmetry. For Nb a mirror-symmetry broken translation state is found as well, which however has a smaller grain displacement and is energetically almost degenerate with a mirror-symmetry conserved translation state. An experimental distinction of this mirror-symmetry broken translation state from a mirror-symmetric one in a Nb bicrystal, e.g. by high-resolution transmission electron microscopy, is difficult if not impossible. For Ta the translation state of the optimized grain-boundary structure is determined to be mirror symmetric. The ab initio results for both Nb and Mo are in close agreement with corresponding experimental observations of Nb and Mo bicrystals by means of high-resolution transmission electron microscopy, showing a conserved and a broken mirror symmetry for Nb and Mo, respectively. Consequently, the ab initio results provide a reliable comparative data base for judgement and improvement of empirical interatomic interaction models suitable for large-scale atomistic simulations of defects in the body-centred cubic transition metals.

Microscopic structure and bonding at the Pd/SrTiO3 (001) interface - An ab-initio local-density-functional study

Abstract The microscopic structure and energetics of a SrTiO3 (001) surface covered with thin layers of Pd have been investigated by means of ab-initio electronic-structure calculations. A mixed-basis pseudopotential technique based on the local density functional theory was employed. Supercells containing SrTiO3 substrate slabs,with either SrO or TiO2 surface terminations, and Pd films of varying thicknesses were used to model the free (001) surfaces and the (001) heterophase interfaces. Based on the calculated energetics of adhesion for the different interfaces, the microscopic energetics of wetting and layer growth has been analysed. The TiO2 terminated substrate is energetically favourable for the adhesion of Pd films, with the Pdatoms bonded on top of the O atoms. The film adhesion is strongest for one (001) layer of fcc Pd and becomes weaker with increasing film thickness.

Prismatic ς 3 (1 0 -1 0) twin boundary in α- Al2O3 investigated by density functional theory and transmission electron microscopy

The microscopic structure of a prismatic Σ3 (1010) twin boundary in α-Al 2 O 3 is characterized by combining ab initio local-density-functional theory, electron energy-loss spectroscopy measuring energy-loss near-edge structures (ELNES) of the oxygen K-ionization edge, and high-resolution transmission electron microscopy (HRTEM). Theoretically, two distinct microscopic boundary variants with very low interface energies are derived and analyzed. The interface-projected densities of states (PDOS) calculated for the two variants agree equally well with ELNES, therefore the comparison between experimental ELNES and theoretical PDOS cannot discriminate the one or the other boundary structure. The analysis reveals that the distinction between the metastable interfaces from ELNES is limited by the spatial resolution of the scanning transmission electron microscope used to measure ELNES, not by its energetical resolution. The quantitative analysis of experimental HRTEM images obtained with an atomic-resolution microscope yields that the experimentally observed interface corresponds to the boundary variant with the lowest energy.

Substitutional cation impurities in α -Al2O3: Ab-initio case study of segregation to the rhombohedral twin boundary

Abstract The energetics of substitutional cation impurities in the bulk volume and at the Σ7 ( 1 012 ) rhombohedral twin boundary in alumina (α-Al 2 O 3 ) were studied by means of ab-initio mixed-basis pseudopotential calculations based on the local density functional theory. For fifteen different substitutional cation impurity species, the positions of the impurity energy levels relative to the valence bands of the oxide were determined, and the type of bonding versus the atomic number of the impurity cation was analysed. For the rhombohedral twin boundary, increased propensity for segregation was found for impurity cations of large ionic radii for which an enhanced overlap between impurity and oxygen electronic states was observed. The effect of structural relaxation at the boundary was studied for the isovalent group-IIIB impurities Sc, Y and La. Segregation broke the screw-rotation symmetry of the pure, unsegregated boundary and altered the interface-plane termination.