Werner Mader

Structural and doping effects in the half-metallic double perovskite A 2 CrWO 6 (A=Sr, Ba, and Ca)

The structural, transport, magnetic, and optical properties of the double perovskite A2CrWO6 with A = Sr,Ba,Ca have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature TC and the saturation magnetization has been determined. A maximum TC = 458 K was found for Sr2CrWO6 having an almost undistorted perovskite structure with a tolerance factor f~1. For Ca2CrWO6 and Ba2CrWO6 structural changes result in a strong reduction of TC. Our study strongly suggests that for the double perovskites in general an optimum TC is achieved only for f~1, that is, for an undistorted perovskite structure. Electron doping in Sr2CrWO6 by a partial substitution of Sr2+ by La3+ was found to reduce both TC and the saturation magnetization Ms. The reduction of Ms could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr2CrWO6 predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in A2CrWO6, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W site positioned in between the high spin Cr sites.

Physics of grain boundaries in the colossal magnetoresistance manganites

Abstract The electrical transport properties of grain boundaries in epitaxial thin films of the perovskite manganites have been studied as a function of temperature and applied magnetic field. Below the Curie temperature T C an additional grain boundary resistance, highly nonlinear current–voltage curves, and a large magnetoresistive effect in the whole temperature regime below T C are found. The results can be explained consistently by the presence of a disordered, a few nm wide paramagnetic grain boundary layer that is depleted below T C due to an increase of the workfunction of the ferromagnetic grain material adjacent to this layer. The related band bending and space charge effects are important for the physics of grain boundaries in the manganites.

TEM investigation on the structure of amorphous silicon monoxide

Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO2. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO2. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 A does not stem from either a-Si or a-SiO2, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO2. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.

Advanced spectroscopic synchrotron techniques to unravel the intrinsic properties of dilute magnetic oxides: the case of Co:ZnO

The use of synchrotron-based spectroscopy has revolutionized the way we look at matter. X-ray absorption spectroscopy (XAS) using linear and circular polarized light offers a powerful toolbox of element-specific structural, electronic and magnetic probes that is especially well suited for complex materials containing several elements. We use the specific example of Zn1−xCoxO (Co:ZnO) to demonstrate the usefulness of combining these XAS techniques to unravel its intrinsic properties. We demonstrate that as long as phase separation or excessive defect formation is absent, Co:ZnO is paramagnetic. We can establish quantitative thresholds based on four reliable quality indicators using XAS; samples that show ferromagnet-like behaviour fail to meet these quality indicators, and complementary experimental techniques indeed prove phase separation. Careful analysis of XAS spectra is shown to provide quantitative information on the presence and type of dilute secondary phases in a highly sensitive, non-destructive manner.

Structure, chemistry and diffusion bonding of metal/ceramic interfaces

Abstract A special technique is used for studying the mechanisms of diffusion bonding between metals and ceramics, particularly at Nb/Al2O3 interfaces. The Nb surfaces are prepared so that arrangements of well defined linear defects, about 10 μm wide and 1 μm deep, are included. Subsequently, bonding is performed under well defined conditions (temperature, time, vacuum, loading, cooling rate, …) and the change of the structure of the flaws is analyzed after decohesion of the partially bonded interface. It can be shown experimentally that a solute solution process of Al2O3 occurs at the interface when sufficiently high temperatures are used. The closing of the flaws depends on (i) the chemical reaction rate, (ii) the diffusion mechanisms in Nb, and (iii) the Al2O3 condensation at interface flaws. The chemical compositions of near-interface regions in Al2O3 and in Nb are determined by analytical electron microscopy. Direct lattice imaging is used for studying the atomic structure near the interface. Facets and dislocations at and near the interface can be identified.

Investigations of the chemistry and bonding at niobiumsapphire interfaces

Spatially resolved electron energy-loss data have been recorded at the interface between niobium and sapphire (α-Al 2 O 3 ), a model metal/ceramic couple. The spatial-difference technique is used to extract interface specific components of the energy-loss near-edge structure (ELNES), which are dependent on the chemistry and bonding across the interface. Multiple scattering calculations of aluminum, oxygen, and niobium clusters were performed to simulate the measured Al L 2,3 ELNES. Two samples fabricated by different techniques were examined. The first interface was made by diffusion bonding pure crystals. Its interface spectrum is identified with tetrahedral coordination of the Al ions at the interface. The calculations match the experimental edge structures, supporting the notion of aluminum to niobium metal bonding and concurring with a structural model in which the basal plane of sapphire at the interface is terminated by a full monolayer (i.e., 67% excess) of aluminum. The second sample was produced by molecular beam epitaxy. The spectrum of this interface is consistent with an atomistic structure in which the interfacial basal plane of sapphire is terminated by oxygen. An unoccupied band of states within the band gap of Al 2 O 3 is observed, signifying chemical bonding between metal and ceramic.

Structural and spectroscopic investigation of (111) twins in barium titanate

Abstract The coherent (111) twin boundary in BaTiO3 has been investigated using quantitative high-resolution transmission electron microscopy and spatially resolved electron-energy-loss spectroscopy (EELS). Lattice images under different defocusing conditions were obtained and compared with simulated images for different structural models. An excellent match between calculated and observed images was obtained only for the model where the Ba-O3 plane constitutes the twin boundary. The oxidation state of Ti at twin boundaries was interrogated by studying the near-edge structure of the L23 ionization edge of Ti using spatially resolved EELS. The difference between spectra taken from the bulk and those including the boundary indicates the presence of Ti in a reduced oxidation state at the boundary. The charge is compensated by oxygen vacancies Vo in the twin plane, which is therefore composed of Ba-O3-x[Vo]x instead of Ba-O3.

STRAIN EFFECTS AND MICROSTRUCTURE OF EPITAXIAL MANGANITE THIN FILMS AND HETEROSTRUCTURES

We have grown epitaxial La2/3Sr1/3MnO3 (LSMO) and La2/3Ba1/3MnO3 (LBMO) thin films as well as La2/3Ba1/3MnO3/SrTiO3 heterostructures by pulsed-laser deposition. The microstructure of the films was analyzed by x-ray diffraction and transmission electron microscopy. A significant effect of strain due to lattice mismatch was found. Whereas the thick LBMO films show perfect epitaxy and grow coherently strained over the full film thickness, the LSMO films were found to be composed of two layers separated by an intrinsic interface region containing a high density of defects. The approximately 60 nm thick bottom layer grows coherently on the SrTiO3 (STO) substrate and is highly strained, whereas the top layer is almost strain free. The LBMO/STO heterostructures are coherently strained and show a very low density of defects and sharp interfaces.

The synthesis of pyrroles via acceptorless dehydrogenative condensation of secondary alcohols and 1,2-amino alcohols mediated by a robust and reusable catalyst based on nanometer-sized iridium particles

Pyrroles are important compounds with several applications in medicine and material science. They can be synthesized sustainably from secondary alcohols and amino alcohols. Hydrogen and water are liberated in the course of this reaction. Here, we present that this sustainable catalytic pyrrole synthesis can be mediated efficiently by a novel iridium nanoparticle catalyst. The catalyst synthesis starts from molecular precursors, an N-ligand stabilized Ir complex and a commercially available polysilazane. The generation of nanometer-sized iridium particles was achieved (due to the presence of N atoms in the support). The robust nature of the support allows reuse of the catalyst. The scope of the reaction was verified by the synthesis of 23 pyrrole derivatives (up to 93% isolated yield). Thus, an attractive functional group tolerance (e.g. amines and olefins) could be observed. Commercially available heterogeneous Ir catalysts are inefficient in this pyrrole synthesis and extremely limited in terms of reusability.

Chemical composition and lattice relaxations at diffusion-bonded Nb/Al2O3 interfaces

Abstract Low-energy Nb/Al2O3 interfaces were prepared by diffusion-bonding single crystals of Nb and sapphire (S) with an orientation relationship (110)Nb|(0001)S and [001]Nb|[01⦶10]S. This orientation relationship was previously observed between Nb and Al2O3 precipitates produced by internal oxidation. Specimens with foil normals parallel to [001]Nb (orientation I) and [1⦶10]Nb (orientation II), suitable for high resolution transmission electron microscopy, were prepared from the bulk bonds. Lattice images in orientation I revealed misfit dislocations in the Nb with a stand-off distance from the interface of 2 to 3 (110)Nb lattice plane spacings. A preferred local matching of [001]Nb atomic columns with respect to the Al2O3 crystal was also observed at this interface. A detailed analysis of the interfacial contrast in orientation II, including image simulations and comparison of observed and calculated contrast, revealed the spacing between the crystals as well as the chemical composition of the interface. The Al2O3 crystal is terminated by an Al layer which contains more Al atoms than an Al layer in Al2O3. The chemical composition is explained by the local activity of O and Al in Nb at near-interface regions, which is established during cooling from the bonding temperature.