J. Roos

NMR, DSC, and conductivity study of a poly(ethylene oxide) complex electrolyte : PEO(LiClO4)x

Abstract The phase diagram of the P(EO) (LiClO 4 ) x system has been determined by DSC and NMR and confirm the existence of two distinct stoichiometric crystalline complexes, the stoichiometry of which are respectively x −1 = 3 and 6. The diffusion constant of Li 7 D has been determined by the Pulsed Magnetic Field Gradient NMR technique on the same membranes studied for conductivity for the three concentrations x = 0.05, 0.167 and 0.125. We deduced that t + the cationic transference number, is temperature independent and equal to 0.18 for x = 0.125 and to 0.23 for x = 0.167

109Ag NMR investigations of the superionic glasses (AgI)x·(Ag2O·2B2O3)1−x

Abstract The first measurements of the 109 Ag NMR relaxation times and linewidths and of the self-diffusion coefficient in ( AgI ) x ·( Ag 2 O ·2 B 2 O 3 ) 1− x are reported. The initial findings indicate that at lower temperatures (which decreases with increasing x) two distinct Ag + populations, mobile and static, can be detected. At high temperatures all cations appear to be mobile. For the x =0.65 sample, the diffusion measurements give an activation energy of 0.36±0.11 eV . The Ag local motions as probed by relaxation and linewidth measurements are associated with activation energies of about 0.10 eV.

NMR study of ion diffusion in the superionic conductor Li3N

Abstract From measurements of the quadrupole splitting and the spin-lattice relaxation times of 7Li NMR the presence of intra- and inter-layer diffusion processes of Li ions together with a liquid-like behaviour at high temperatures have been deduced. Activation energies and anomalous high “attempt” frequencies are obtained. Anisotropic relaxation rates reflect dimensionality effects of diffusion.

Anisotropy of the superconducting state properties and phase diagram of MgB2 by torque magnetometry on single crystals

Abstract The angular and temperature dependence of the upper critical field H c2 in MgB 2 was determined from torque magnetometry measurements on single crystals. The H c2 anisotropy γ H was found to decrease with increasing temperature, in disagreement with the anisotropic Ginzburg–Landau theory, which predicts that γ H is temperature independent. This behaviour can be explained by the two band nature of superconductivity in MgB 2 . An analysis of measurements of the reversible torque in the mixed state yields a field dependent effective anisotropy γ eff , which can be at least partially explained by different anisotropies of the penetration depth and the upper critical field. It is shown that a peak effect in fields of about 0.85 H c2 is a manifestation of an order–disorder phase transition of vortex matter. The H – T phase diagram of MgB 2 for H ∥ c correlates with the intermediate strength of thermal fluctuations in MgB 2 , as compared to those in high and low T c superconductors.

Ag diffusion constant in RbAg4I5 and KAg4I5 determined by pulsed magnetic gradient NMR

We have measured the diffusion constant D of 109Ag in the temperature range 150 to 300 K in RbAg4I5 and KAg4I5 by nuclear magnetic resonance employing the pulsed magnetic gradient method. The activation energies are 0.12 ± 0.01 eV for the α phase in both compounds and 0.16±0.01 eV and 0.18±0.01 eV for the β phases of RbAg4I5 and KAg4I5, respectively. At least in KAg4I5 our data reveal a discontinuous change of D at the α→β transition. In terms of the Haven ratio a decrease from 0.51 to 0.39 is observed for the α→β transition. This decrease is taken as evidence for an increased correlation of the Ag motion in the β phase.

The interdependence of mobile and stationary ions in RbAg4I5— An NMR investigation

Abstract Previous NMR studies in RbAg4I5 have been supplemented by additional measurements on the 87Rb nuclei and extended to the mobile Ag ion. The thermally activated relaxation rates of the stationary and the mobile ions show remarkable differences in the overall temperature behavior as well as at the 208 K phase transition. A possible interpretation is given in terms of “trial and error” hopping and configurational response of the mobile Ag ions.

Charge-excitation picture of Cu NMR Knight shift and relaxation in YBa2Cu4O8 deduced from a 3-band Hubbard model

A simplified model of copper hole excitations in the singlet band of YBa2Cu4O8 is presented. The starting point is a three-band Hubbard model comprising a Kondo-like exchange between the copper and oxygen holes which gives rise to a new singlet correlated band. This band corresponds to the motion of Zhang-Rice singlets through the antiferromagnetic background of localized copper spins and exhibits a well-developed saddle point of strong copper character. The main contribution to the susceptibility comes from thermally activated copper holes in the extended saddle point of the singlet band. From an idealization of the corresponding density of states we derive formulae for the spin-lattice relaxation time T1 and the Knight shift K at the Cu(2) site.

Dimensionality effects of Li diffusion in β-eucryptite studied by single crystal Li NMR

Abstract We have studied the quadrupole splittings and relaxation rates of 7 Li and 6 Li in single crystals of LiAlSiO 4 from 77K to 760K. The data reveal two diffusion processes: 1D diffusion along the main and secondary channels and hopping of Li ions betwen different channels. Some samples intentionally doped with Fe-ions are used to check theoretical predictions about relaxation by 1D diffusion to paramagnetic impurities.

Cooperative lithium motion and NMR relaxation of 7Li by paramagnetic impurities in β-eucryptite (LiAlSiO4)

Abstract Evidence for one-dimensional (1D) cooperative diffusion of Li ions along the structural channels is given by explaining the observed fine structure of the 7 Li NMR central signal as the result of nuclear dipolar coupling in a linear chain of 7 Li nuclei. The relaxation of 7 Li by Fe impurities has been measured below 400 K. At 300 K, the relaxation rate varies faster than linear with the concentration and is roughly proportional to the inverse NMR frequency. The data are discussed in terms of a relaxation model which incorporates 3D spin diffusion to a barrier surrounding the impurity together with further 1D particle hopping inside the barrier.

Detection of a second Br site in K2Pt(CN)4Br0.3 3.2 H2O (KCP) by 18Br and 39K NMR

Abstract By nuclear magnetic resonance we have detected between 20 and 300 K two bromine signals with a temperature-independent intensity ratio of 2.5 ± 0.3 proving the existence of a second Br site in KCP. The result is supported by the observation of three potassium signals. Thus, three types of unit cells exist: 40% contain defect water, 43% have Br in site I and 17% Br in site II.