Study of oxygen ordering in htc superconductors by magnetic resonance of different nuclei

NMR of 169Tm and NQR/NMR of 63/65Cu was applied to investigate the order of oxygen in the HTC superconductors of the type RBa2Cu301. From the spectra of three different lattice sites, one for Tm and two for Cu, it is evident, that three ordered structures exist: Tetra for x = 6.0, ortho-II for x = 6.5 and ortho-I for x = 7.0. No other ordered structures could be detected. A coexistence of ortho-I and -II is evident in a wide region between 6.6 and 6.8. The existence of extended domains of ortho-II is essential for the onset of superconductivity. In the tetragonal region below 6.3 we find mainly insulated oxygen ions in the Cu(1) plane; in superconducting samples of the 60 K plateau oxygen is ordered in chains of a mean length of about 12 oxygen ions. In the region of the 90 K plateau the chains become very long and the oxygen vacancies are clustered in empty fragments of chains. © 1993 IOP Publishing Ltd

Molecular orientational dynamics of the endohedral fullerene Sc3_{3}N@C80_{80} as probed by 13^{13}C and 45^{45}Sc NMR

We measure 13C and 45Sc NMR lineshapes and spin-lattice relaxation times (T1) to probe the orientational dynamics of the endohedral metallofullerene Sc3N@C80. The measurements show an activated behavior for molecular reorientations over the full temperature range with a similar behavior for the temperature dependence of the 13C and 45Sc data. Combined with spectral data from Magic Angle Spinning (MAS) NMR, the measurements can be interpreted to mean the motion of the encapsulated Sc3N molecule is independent of that of the C80 cage, although this requires the similar temperature dependence of the 13C and 45Sc spin-lattice relaxation times to be coincidental. For the Sc3N to be fixed to the C80 cage, one must overcome the symmetry breaking effect this has on the Sc3N@C80 system since this would result in more than the observed two 13C lines.Comment: 6 pages, 5 figure

Jahn-Teller orbital glass state in the expanded fcc Cs3C60 fulleride

The most expanded fcc-structured alkali fulleride, Cs3C60, is a Mott insulator at ambient pressure because of the weak overlap between the frontier t1u molecular orbitals of the C603− anions. It has a severely disordered antiferromagnetic ground state that becomes a superconductor with a high critical temperature, Tc of 35 K upon compression. The effect of the localised t1u3 electronic configuration on the properties of the material is not well-understood. Here we study the relationship between the intrinsic crystallographic C603− orientational disorder and the molecular Jahn–Teller (JT) effect dynamics in the Mott insulating state. The high-resolution 13C magic-angle-spinning (MAS) NMR spectrum at room temperature comprises three peaks in the intensity ratio 1:2:2 consistent with the presence of three crystallographically-inequivalent carbon sites in the fcc unit cell and revealing that the JT-effect dynamics are fast on the NMR time-scale of 10−5 s despite the presence of the frozen-in C603− merohedral disorder disclosed by the 133Cs MAS NMR fine splitting of the tetrahedral and octahedral 133Cs resonances. Cooling to sub-liquid-nitrogen temperatures leads to severe broadening of both the 13C and 133Cs MAS NMR multiplets, which provides the signature of an increased number of inequivalent 13C and 133Cs sites. This is attributed to the freezing out of the C603− JT dynamics and the development of a t1u electronic orbital glass state guided by the merohedral disorder of the fcc structure. The observation of the dynamic and static JT effect in the Mott insulating state of the metrically cubic but merohedrally disordered Cs3C60 fulleride in different temperature ranges reveals the intimate relation between charge localization, magnetic ground state, lifting of electronic degeneracy, and orientational disorder in these strongly-correlated systems

Two-dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2: phase transitions and AFMorder as seen by 87Rb NMR

We report the Rb nuclear magnetic resonance (NMR) results in a recently synthesized Na5RbCu4(AsO4)Cl2. This complex novel two-dimensional (2D) cuprate is an unique magnetic material, which contains layers of coupled Cu4O4 tetramers. In zero applied magnetic field, it orders antiferromagnetically via a second-order low-entropy phase transition at TN = 15(1) K. We characterise the ordered state by 87Rb NMR, and suggest for it a noncollinear rather than collinear arrangement of spins. We discuss the properties of Rb nuclear site and point out the new structural phase transition(s) around 74 K and 110 K.Comment: 2 pages, 2 figures, Proceedings of SCES'05, Vienna 200

Modelling of an aza-Michael reaction from crystalline naphthalene derivatives containing peri–peri interactions: very long N–C bonds?

The separation between a pair of peri-located dimethylamino and ethene-2,2-dinitrile groups in a naphthalene molecule, which models the progress of a Michael reaction, can be controlled by the installation of a short ethylene bridge or the introduction of repulsive interactions at the opposite set of peri positions. Introduction of a dimethylammonium substituent produced a hydrated chloride salt in which the Me2N⋯C(H)[double bond, length as m-dash]C(CN)2 separation between reactive groups decreases, reversibly, from 2.167 Å at 200 K to 1.749 Å at 100 K, with the maximum rate of change in the range 128–140 K, which was studied by variable temperature X-ray crystallography and solid state NMR. From these and other crystallographic data a correlation between Me2N⋯C bond formation and alkene bond breaking was constructed for the first step of an aza-Michael reaction

Magnetic Field-Induced Condensation of Triplons in Han Purple Pigment BaCuSi2_2O6_6

Besides being an ancient pigment, BaCuSi$_2$O$_6$ is a quasi-2D magnetic insulator with a gapped spin dimer ground state. The application of strong magnetic fields closes this gap creating a gas of bosonic spin triplet excitations called triplons. The topology of the spin lattice makes BaCuSi$_2$O$_6$ an ideal candidate for studying the Bose-Einstein condensation of triplons as a function of the external magnetic field, which acts as a chemical potential. In agreement with quantum Monte Carlo numerical simulations, we observe a distinct lambda-anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid Helium temperatures.Comment: published on August 20, 200

Proton relaxation in the superconducting organic solid (BEDT-TTF)2Cu(NCS)2: evidence for relaxation by localized paramagnetic centers

The nonexponential character of the build-up of the nuclear magnetization usually increases on lowering the temperature. Superconducting fluctuations have been offered in a speculative way as an explanation for the nonexponential spin relaxation. We have observed the same relaxation behaviour in our investigation of the proton spin relaxation in (BEDT-TTF)2Cu(NCS)2 which is an organic conductor at room temperature and becomes a superconductor at Tc=10.4 K at ambient pressure. As we noticed during preliminary measurements that the degree of deviation from exponential relaxation depends on the sample under study we decided to do experiments under controlled conditions of sample preparation

Determination of acid sites in porous aluminosilicate solid catalysts for aqueous phase reactions using potentiometric titration method

A potentiometric acid base titration method was developed to characterize the acid base properties of H-Beta-25, H-Beta-300, H-Ferrierite-20 and Si-MCM-48 zeolites in aqueous solutions. The method is based on linearization of the titration curve. The zeolite powders were dispersed in a sodium nitrate solution which was then titrated with a standard solution of sodium hydroxide. Various acid sites were found in the studied zeolites, and both the protonation constants and the concentrations of these acid sites were determined. The chemical composition (aluminum and silicon), distribution of the aluminum sites, crystallinity as well as the acidic properties of the titrated zeolites were compared with those of the pristine zeolites by using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), X-ray Diffraction (XRD), Al-27 NMR spectroscopy and the commonly used gas phase Fourier Transform Infrared spectroscopy (FTIR) of pyridine adsorption method. (C) 2015 Elsevier Inc. All rights reserved.</p