F. Qi

Stimulated echo NMR spectroscopy of slow ionic motions in a solid electrolyte

Abstract Lithium spin-alignment spectroscopy is presented as an NMR technique for studying slow translational motions in solid and solid-like ionic conductors. We employ phase cycling that allows to measure two-time translational correlation functions via the generation of a pure quadrupolar ordered state. Correlation functions of the crystalline electrolyte Li 3 Sc 2 (PO 4 ) 3 were recorded for times ranging from about 0.1 ms to more than 10 s, implying that translational diffusion coefficients smaller than 10 −20 m 2 /s become accessible.

Nuclear magnetic resonance and dielectric spectroscopy of a simple supercooled liquid: 2-methyl tetrahydrofuran

The small-molecule glass former methyl tetrahydrofuran (MTHF) was investigated using dielectric spectroscopy, spin-lattice relaxometry, multidimensional stimulated-echo nuclear magnetic resonance techniques, and field gradient diffusometry. We show experimentally that MTHF nicely fits into the pattern of related small-molecule glass-forming liquids, including the existence of a high-frequency contribution to the dielectric loss, the appearance of a pronounced translational enhancement, the dominance of small average rotational jump angles, and the existence of short-lived dynamical heterogeneity.

Structural relaxation of the fragile glass-former propylene carbonate studied by nuclear magnetic resonance

Proton, deuteron, and carbon NMR methods were applied to study various isotopic species of supercooled and glassy propylene carbonate. The molecular diffusion was investigated via static field gradient diffusometry and yielded evidence for a particularly pronounced enhancement of the translational over the reorientational dynamics. The latter was monitored using measurements of spin-lattice relaxation times and of two-time stimulated echoes. The sin–sin correlation functions exhibited a pronounced dependence on the evolution times. From these results it is concluded that the geometry of molecular reorientation can be described as a superposition of moderately small jump angles (about 30°) and a quasi-diffusive component which is responsible for about one-third of the total angular displacement. Finally, four-time stimulated echoes confirmed that the nonexponential relaxation in propylene carbonate is dynamically heterogeneous in nature.

Slow and fast methyl group rotations in fragile glass-formers studied by NMR

Abstract The spin-lattice relaxation times of the selectively ring deuterated, fragile glass-formers propylene carbonate and toluene were compared with those measured for species which were specifically labeled at the methyl groups. It was found that the dynamics of the CD 3 group is strongly decoupled from that associated with the primary response of toluene, while for propylene carbonate the degree of decoupling is relatively weak. The experimental results could be described successfully using a model which takes into account the ring dynamics as well as those of the methyl group.

Complex hopping dynamics in the deuteron conductors K3D(SO4)2 and Rb3D(SO4)2

The translational deuteron motions in the paraelectric phases of K3D(SO4)2 and Rb3D(SO4)2 were studied using two-dimensional exchange nuclear magnetic resonance, performed in the frequency as well as in the time domains. These experiments reveal that in Rb3D(SO4)2 the ion hopping is about one order of magnitude slower than in K3D(SO4)2. Additional measurements of the spinlattice relaxation times were carried out at the proton, deuteron, and rubidium sites. It was found that the relaxation times determined for deuterated as well as for protonated tri-rubidium-hydrogen-disulfates are a factor of ten longer than those for the corresponding tri-potassium-hydrogen-disulfates. The results reveal that the deuteron hopping is to be regarded as a two-step process involving a coupling to rotational or librational motions of the sulfate tetrahedra.