R. Podloucky

Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations

Abstract Atomically resolved scanning tunneling microscopy (STM) of ultra-thin NaCl films on Al(111) and Al(100) demonstrates that only one atomic species of NaCl is imaged as a protrusion. By comparison of the constant current STM images with ab initio calculations of the local density of states by means of the full-potential linearized augmented plane wave method, the protrusions could be attributed to the anion Cl – . The calculations shows that a higher density of occupied states at the Cl-sites than for the Na-sites around the Fermi level causes the STM contrast between Cl and Na. With increasing number of NaCl layers the density of states in the band gap is reduced and the apparent height of additional NaCl layers decreases. The maximum film thickness allowing successful imaging by STM was found to be three layers.

Vacancy induced changes in the electronic structure of titanium carbide—I. Band structure and density of states

Abstract Results of a self-consistent “Augmented Plane Wave” (APW) band-structure calculation are presented for substoichiometric titanium carbide with 25% vacancies on the carbon sublattice sites (TiC 0 75 ) assuming a model structure with ordered vacancies Comparison with an earlier APW study on stoichiometric TiC reveals that the carbon vacancies induce two pronounced peaks in the density of states (DOS), 0.4 eV below and 0.8 eV above the Fermi energy E γ , thus forming electronic states in a region where the DOS for stoichiometric TiC exhibits a minimum So-called “vacancy states” with an important amount of charge on the vacancy site are found to be derived from Ti 3 d states extending into the vacancy muffin tin sphere An angular momentum decomposition with respect to the center of the vacancy muffin tin sphere shows that the s character predominates for the occupied and the p character for the unoccupied “vacancy states” The theoretical findings explain features near E γ , observed in recently published X-ray emission spectra Furthermore, we find a slight increase of electronic charge in the carbon muffin tin spheres as compared with stoichiometnc TiC.

Platinum nanocrystals supported by silica, alumina and ceria: metal–support interaction due to high-temperature reduction in hydrogen

Regular Pt nanoparticles, obtained by epitaxial deposition on NaCl surfaces, were supported by thin films of silica, alumina and ceria and sub- jected to hydrogen reduction at temperatures up to 1073 K. The changes in morphology and composition were followed by (HR)TEM, electron diffraction and EELS, and the results were supported by theoretical calculations. The structural changes of the Pt particles upon reduction at 773 K and above are surprisingly similar despite the differing chemical properties of the three supports. Some platelet- and cube-like geometries exhibit double lattice periodicities in high resolution images and electron diffraction patterns. With increasing reduction larger aggregates of more complex appearance and structure are formed. Surface reconstruction under hydrogen and alloy formation are considered as responsible for this effect. Most likely, the first step is identical on all three systems and consists in the topotactic formation of Pt rich Pt3Me (Me = Si, Al, Ce) under the influence of hydrogen, followed by transformation into diverging structures of lower Pt content and different crystallography. Density func- tional calculations were performed for deriving energies of formation of PtMe and Pt 3Me compounds.

Superconductivity in novel Ge-based skutterudites: {Sr,Ba}pt4Ge12.

Combining experiments and ab initio models we report on SrPt4Ge12 and BaPt4Ge12 as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge atoms. Below T(c)=5.35 and 5.10 K for BaPt4Ge12 and SrPt4Ge12, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-p states dominate the electronic structure at the Fermi energy.

Atomic modelling of Nb, V, Cr, and Mn substitutions in γ-TiAl. I: c/a ratio and site preference

Abstract The Linear Muffin Tin Orbital Method was applied to calculate from first principles the electronic structure and total energies of L 1 0 ordered TiXAl 2 and Ti 2 XAl ( X = Nb , V , Cr , Mn ) compounds. Volumes and lattice parameters were determined by minimizing total energies. If X substitutes for Ti, the ratio c/a of the lattice parameters is always slightly larger than for the reference compound γ-TiAl. In the case of Al substitutions, c/a decreases substantially for all X. In general, the Ti-rich Ti 2 XAl compounds have larger unit cell volumes than the Al-rich counterparts. For any particular X, the interatomic distances between X and Ti appear to be rather constant, irrespective of the chosen substitution site. These two findings are sufficient to explain the substantial reduction of c/a for the Ti-rich (or Al-substituted) cases obtained by the calculations. From the formation energies, Δ E , the site preference was derived by E site = ΔE Al - rich − ΔE Ti - rich combining site preferences and c/a changes, we found that Nb and V prefer the Ti sites whereas Cr and Mn tend to substitute for Al. We conclude that X = Nb or V slightly increases c/a whereas X = Cr or Mn decreases this ratio. Although our calculations were made for 25.at% substitutions, many of our results are in agreement with experimental data for alloys with a small percentage of x.

Atomic modelling of Nb, V, Cr and Mn substitutions in γ-TiAl. 2: Electronic structure and site preference

Abstract The effect of Nb, V, Cr and Mn on the lattice parameters of γ-TiAl and the site preference of these transition metals were studied based on first-principles supercell calculations of the electronic structure and total energy of ordered Ti n − 1 XAl n and Ti n XAl n − 1 compounds with X = Nb, V, Cr and Mn. For the calculation of optimized volumes by total energy minimization 4-, 8- and 32-atom cells were taken into account. Trends of c a changes in connection with the analysis of the electronic structure are derived from 4-atom supercell results in continuation of a previously published Part 1 of our investigations. We find that Mn has the strongest preference for Al sites because of strong stabilizing electronic structure effects. The substantial decreased value of the c a ratio found for the case of Mn substituting for Al is related to a bcc-like local arrangement of Ti around the Mn atom. This geometrical situation is accompanied by the formation of a deep pseudogap in the density of states. The Fermi energy falls precisely into this pseudogap which distinctly separates TiMn bonding and antibonding states. The site preference trend for the other substitutional compounds containing X = Nb, V, Cr are also discussed in combination with their corresponding density of states.

FLAPW: applications and implementations.

Modern material design involves a close collaboration between experimental and computational materials scientists. To be useful, the theory must be able to accurately predict the stability and properties of new materials, describe the physics of the experiments, and be applicable to new and complex structures-the all-electron full-potential linearized augmented plane wave (FLAPW) is one such method that provides the requisite level of numerical accuracy, albeit at the cost of complexity. Technical aspects and modifications related to the choice of basis functions (energy parameters, core-valence orthogonality, extended local orbitals) that affect the applicability and accuracy of the method are described, as well as an approach for obtaining k-independent matrix elements. The inclusion of external electric fields is illustrated by results for the induced densities at the surfaces of both magnetic and non-magnetic metals, and the relationship to image planes and to nonlinear effects such as second harmonic generation. The magnetic coupling of core hole excitations in Fe, the calculation of intrinsic defect formation energies, the concentration-dependent chemical potentials, entropic contributions, and the relative phase stability of Zr-rich Zr-Al alloys are also discussed.

Computational and experimental study of phase stability, cohesive properties, magnetism and electronic structure of TiMn2

Abstract By an ab initio approach we calculated phase stability, cohesive and magnetic properties, and the electronic structure of TiMn 2 for the C14 and C15 Laves structure types. The nonmagnetic C14 phase is the ground state in accordance to experiment, whereas a metastable ferromagnetic C15 phase is predicted with a local magnetic moment of 0.78 μ β for Mn. The energy of formation was measured by a calorimetric drop experiment resulting in a value of −86.76±6.79 kJ mol −1 at 298 K being in good agreement to the ab initio result of −88.8 kJ mol −1 . Model calculations based on Miedema’s approach failed to yield reasonable results. The calculated densities of states reveal strong hybridisation between Ti-like and Mn d-like states.

The effect of vacancies on the electronic structure of Nbo

Abstract The crystal structure of NbO is considered as a NaCl structure with 25% ordered vacancies in each sublattice. Self-consistent energy band structure calculations using the augmented plane wave (APW) method have been carried out for a hypothetical NbO with the full NaCl structure and for the structure with the vacancies. We find that the introduction of the vacancies leads to (1) a lowering of the Nb- d like bands with respect to the O- p bands, (2) an almost completely filled “vacancy band”, and (3) a splitting and broadening of the O- p like band. These effects are shown to have a significant influence on both the theoretically calculated X-ray photoemission spectra (XPS) and the X-ray emission spectra (XES). All theoretical spectra obtained with the vacancy-structure are found to be in good agreement with experiments in contrast to those spectra calculated for hypothetical NbO in the NaCl structure.

Scanning tunneling microscopy of binary-alloy surfaces: is chemical contrast a consequence of alloying?

Abstract Recent STM studies achieved chemical resolution on PtRh and PtNi alloy surfaces. By a first-principles method employing the Tersoff–Hamann model, we have simulated STM scans on PtRh and PtNi(100) surfaces by calculating the apparent heights of individual surface atoms. The difference in apparent heights between Pt and Rh atoms is caused by changes in the density of states due to alloying. The simulations for the PtNi(100) surface, however, yield apparent heights of Pt and Ni atoms below atomic resolution, indicating that in the experiment, tip–sample interactions are responsible for chemical and atomic resolution.