90 research outputs found

    Reconciling local structure disorder and the relaxor state in (Bi1/2Na1/2)TiO3 -BaTiO3

    Get PDF
    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 Hg0.8_{0.8}Cu0.2_{0.2}Ba2_2Ca2_2Cu3_3O8+δ_{8+\delta} below TcT_c seen by NQR

    Full text link
    We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer high-Tc T_{c} -compounds HgBaCaCuO and HgBaCaCuO(F) with TcT_c 134 K. Above Tc T_{c} two Cu NQR line pairs are observed in the spectra corresponding to the two inequivalent Cu lattice sites. Below Tc T_{c} 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 63/65^{63/65}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 63/65^{63/65}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

    NQRS Data for Al10Cu20Ni8Ti3Zr59 (Subst. No. 0110)

    No full text

    Solid-State NMR Characterization of Wilkinson's Catalyst Immobilized in Mesoporous SBA-3 Silica

    No full text
    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

    Formation of magnetic moments in the cuprate superconductor Hg 0.8Cu0.2Ba2Ca2Cu3O 8+δ below Tc seen by NQR

    No full text
    We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer high-Tc-compounds HgBaCaCuO and HgBaCaCuO(F) with Tc = 134 K. Above Tc two Cu NQR line pairs are observed in the spectra corresponding to the two inequivalent Cu lattice sites. Below Tc 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 levels indicating that the additional lines are due to a Zeemann splitting of the 63/65Cu 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 63/65Cu 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 c-axis with magnitudes of the order of 1000 G. We also discuss the possible microscopic origin of the observed internal magnetic fields. © 2004 Elsevier B.V. All rights reserved
    corecore