Hydrogen bonds and local symmetry in the crystal structure of gibbsite

First‐principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the ²⁷Al MAS NMR resonances in gibbsite. The ²⁷Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished ²⁷Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al‐I site characterized by a C_Q = 4.6 MHz is surrounded by OH? groups participating in four intralayer and two interlayer hydrogen bonds, while the Al‐II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH‐bonds are aligned toward the interlayer gallery. In high‐resolution solid‐state ¹H CRAMPS (combination of rotation and multiple‐pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite‐like layer—intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright © 2010 John Wiley & Sons, Ltd.     

Core-level photoelectron study of indium chains on Si(1 1 1) at 10 K

High-resolution core-level data from the prototypical In/Si(1 1 1) system have been acquired at 10 K. An asymmetric tail in the In 4d spectra reveals a metallic character of the low temperature Si(1 1 1)8 × 2 phase confined to the inner indium rows. The decoupling of the one-dimensional inner indium chains from any metallic environment at ∼10 K suggests a possible Luttinger liquid behavior. At room temperature essentially a broadening of the spectral features is noticed, which appears compatible with a fluctuation scenario.     

Mn site substitution of La0.67Ca0.33MnO3 with closed shell ions: Effect on magnetic transition temperature

Mn site is substituted with closed shell ions (Al, Ga, Ti, Zr and a certain combination of Zr and Al) and also with Fe and Ru ions carrying the magnetic moment (S=5/2 and 2 respectively) at a fixed concentration of 5 at %. Substitution did not change either the crystal symmetry or the oxygen stoichiometry. All substituents were found to suppress both the metal-insulator and ferromagnetic transition temperatures (T _p(ρ) and T _C, respectively) to varied extents. Two main contributions identified for the suppression are the lattice disorder arising due to difference in the ionic radii between the substituent (r _M) and the Mn³⁺ ion (r _Mn ³⁺) and in the case of the substituents carrying a magnetic moment, the type of magnetic coupling between the substituent and that of the neighboring Mn ion.     

k Dependence of the crystal-field splittings of 4f states in rare-earth systems

The occupation, energy separation, and order of the crystal-field-split 4f states are crucial for the understanding of the magnetic properties of rare-earth systems. We provide the experimental evidence that crystal-field-split 4f states exhibit energy dispersion in momentum space leading to variations of energy spacings between them and even of their energy sequence across the Brillouin zone. These observations were made by performing angle-resolved photoemission experiments on YbRh(2)Si(2) and properly simulated within a simple model based on results obtained by inelastic neutron scattering experiments and band structure calculations. Our findings should be generally applicable to rare-earth systems and have considerable impact on the understanding of magnetism and related phenomena.     

High-resolution Russian–German beamline at BESSY

The superior performance of the high-resolution Russian–German bending-magnet beamline at BESSY is demonstrated by measuring photon flux and photoionization spectra in the spectral region of the 2,-1₃ resonance of doubly-excited He as well as at the 1s absorption thresholds of gas-phase N₂, O₂, and Ne. It is shown that even after 6 years of successful operation, the beamline remains to provide a solid basis for a further development of the Russian–German scientific cooperation in the field of applications of synchrotron radiation.     

The electronic structure of cobalt phthalocyanine

The electronic structure and morphology of organic semiconducting cobalt-phtalocyanine (CoPc) films in situ prepared on the Au(001)-5×20 superstructure have been studied by a combination of experimental and theoretical work. The CoPc molecular film was characterized by photoemission spectroscopy (PES, valence band and core-level). The experimental results were simulated and have been explained in the framework of density functional theory (DFT) calculations. The C 1s and N 1s core level spectra were analyzed by taking into account the fact that both types of atoms have different nonequivalent positions in the molecule. And finally, the experimentally obtained electronic valence band structure of CoPc is in very good agreement with ab initio density of state results, allowing a detailed site-specific insight into the system.     

Silver on copper phthalocyanine: Abrupt and inert interfaces

The chemistry and electronic structure evolution were studied upon the formation of the interfaces between copper phthalocyanine and Ag and Sn deposited on it in ultra high vacuum conditions. The measurements were performed by means of high-resolution photoemission spectroscopy of core-levels and the valence band, and using synchrotron-radiation facilities. The investigations demonstrate the formation of chemically inert and abrupt interfaces.     

Electronic structure of the organic semiconductor copper phthalocyanine: experiment and theory

The electronic structure of the organic semiconductor copper-phthalocyanine (CuPc) has been determined by a combination of conventional and resonant photoemission, near-edge x-ray absorption, as well as by the first-principles calculations. The experimentally obtained electronic valence band structure of CuPc is in very good agreement with the calculated density of states results, allowing the derivation of detailed site specific information.     

Fermi gap stabilization of an incommensurate two-dimensional superstructure

The compressed, incommensurate approximately (9.5 x 9.5) moire superstructure of the Ag monolayer on Cu(111) displays a filled surface state band with a Fermi energy gap at the Brillouin zone boundary. By contrast, the surface band is gapless for the less compressed, commensurate (9 x 9) moire of two Ag layers. A simple estimate of the energy gain rendered by opening this gap gives a value similar to the elastic energy change required to modify the commensurate structure, thereby suggesting that the approximately (9.5 x 9.5) incommensurate phase is stabilized by such a gap opening. The possible presence of a charge density wave state is discussed.