90 research outputs found
Reconciling local structure disorder and the relaxor state in (Bi1/2Na1/2)TiO3 -BaTiO3
Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi1/2Na1/2)TiO3-xBaTiO(3) (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (Delta QIS(width)) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in Na-23 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.ope
Evidence for the formation of magnetic moments in the cuprate superconductor HgCuBaCaCuO below seen by NQR
We report pure zero field nuclear magnetic resonance (NQR) measurements on
the optimally doped three layer high--compounds HgBaCaCuO and
HgBaCaCuO(F) with 134 K. Above two Cu NQR line pairs are
observed in the spectra corresponding to the two inequivalent Cu lattice sites.
Below the Cu NQR spectra show additional lines leading to the extreme
broadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. The
spin-lattice relaxation curves follow a triple exponential function with
coefficients depend onto the saturation time (number of saturation pulses),
whereas the spin-spin relaxation curve is described by a single exponential
function. From the spin-lattice relaxation we deduced a complete removal of the
Kramers degeneracy of the Cu quadrupole indicating that the additional lines
are due to a Zeemann splitting of the Cu lines due to the spontaneous
formation of magnetic moments within the CuO layers. Below 140 K, the spectra
are well fitted by a number of 6 Cu line pairs. From the number of
the Cu lines, the position of the lines relative to each other and the complete
removal of the Kramers degeneracy we deduced an orientation of the magnetic
moments parallel to the symmetry axis of the electric field gradient tensor
with magnitudes of the order of 1000 G. We also discuss the possible
microscopic origin of the observed internal magnetic fields.Comment: 11 pages, 12 figure
Sedimenttransport durch tropische Wirbelströme im submarinen Delta des Ganges-Brahmaputra/Bangladesh
Solid-State NMR Characterization of Wilkinson's Catalyst Immobilized in Mesoporous SBA-3 Silica
The Wilkinson's catalyst RhCl(PPh3)(3) has been immobilized inside the pores of amine functionalized mesoporous silica material SBA-3 and The structure of the modified silica surface and the immobilized rhodium complex was determined by a combination of different solid-state NMR methods. The successful modification of the silica surface was confirmed by Si-29 CP-MAS NMR experiments. The presence of the T-n peaks confirms the successful functionalization of the support and shows the way of binding the organic groups to the surface of the mesopores. P-31-P-31 J-resolved 2D MAS NMR experiments were conducted in order to characterize the binding of the immobilized catalyst to the amine groups of the linkers attached to the silica surface. The pure catalyst exhibits a considerable P-31-P-31 J-coupling, well resolvable in 2D MAS NMR experiments. This I-coupling was utilized to determine the binding mode of the catalyst to the linkers on the silica surface and the number of triphenylphosphine ligands that are replaced by coordination bonds to the amine groups. From the absence of any resolvable P-31-P-31 J-coupling in off-magic-angle-spinning experiments. as well as slow-spinning MAS experiments, it is concluded, that two triphenylphosphine ligands are replaced and that the catalyst is bonded to the silica surface through two linker molecules
Efficient analysis of V-51 solid-state MAS NMR spectra using genetic algorithms
A program for iterative fitting procedures to determine the NMR parameters from V-51 solid-state MAS NMR spectra was developed. it contains options to use genetic algorithms and downhill-simplex optimizing procedures to extract the optimal parameter sets, which describe our spectra. As computational kernel the SIMPSON program is employed. Other kernels like SPINEVOLUTION are easily incorporable. The algorithms are checked for their suitability for the present optimization problem and optimal simulation conditions are determined, with the focus on minimal processing time. The procedure leads to a very good agreement between experimental and simulated spectra in a passable period of time. First results for spectra of model compounds for the active site of vanadium haloperoxidases are presented. (C) 2008 Elsevier Inc. All rights reserved
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