Origin of non-exponential relaxation in a crystalline ionic conductor: a multi-dimensional 109Ag NMR study

The origin of the non-exponential relaxation of silver ions in the crystalline ion conductor Ag7P3S11 is analyzed by comparing appropriate two-time and three-time 109Ag NMR correlation functions. The non-exponentiality is due to a rate distribution, i.e., dynamic heterogeneities, rather than to an intrinsic non-exponentiality. Thus, the data give no evidence for the relevance of correlated back-and-forth jumps on the timescale of the silver relaxation.Comment: 4 pages, 3 figure

Solid-State Dynamics in the closo-Carboranes:A (11)B MAS NMR and Molecular Dynamics Study

This work explores the dynamic behaviour of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the 11B MAS linewidths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (tauc) calculated by molecular dynamics are on the picosecond timescale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the 11B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the 11B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQeff derived from “dynamic shift” measurements using 11B MQMAS NMR spectroscopy. The behaviour of PQeff as a function of temperature for para-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures and this was supported by the molecular dynamics simulations

Phase transition kinetics and reorientational dynamics of the plastic crystal meta-carborane studied by deuteron NMR

Meta-carborane shows an orientationally disordered, orthorhombic phase which transforms into an ordered monoclinic phase near 170 K and into a quasi-isotropic cubic phase near 280 K. From deuteron magnetization measurements both transitions are found to exhibit a thermal hysteresis and slow transformation kinetics. Furthermore, solid-echo NMR spectra and spin-lattice relaxation times are reported. These results indicate that in the orthorhombic phase the meta-carborane molecules perform a strongly anisotropic motion which is characterized by a temperature dependent amplitude

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

Hysteresis effects and mesoscopic domains: Precursors for the formation of orientational glass states

${\rm (NH_4I)}_x{\rm (KI)}_{1-x}$ single crystals were investigated using proton NMR for 0.8 < x < 1. They undergo a first-order transition from the orientationally disordered high-temperature α-phase into the low-temperature β-phase. This transition exhibits a thermal hysteresis which strongly increases upon lowering x. In the $\alpha,\beta$ coexistence range evidence for nanoscopic domains was obtained using proton spin diffusion. The characteristic dimensions of the domains decrease as the critical concentration xc is approached, below which an orientational glass state is stable at low temperature. This observation indicates that close to xc the ${\rm (NH_4I)}_x{\rm (KI)}_{1-x}$ orientational glasses are to be characterized as nano domain states

Rotational motion in the molecular crystals meta- and ortho-carborane studied by deuteron nuclear magnetic resonance

Spin-lattice and spin-spin-relaxation times, one- and two-dimensional spectra as well as two- and four-time correlation functions were measured for the molecular crystals ortho- and meta-carborane using deuteron nuclear magnetic resonance. It is found that in their noncubic phases these crystals exhibit highly anisotropic motions. In order to allow for a quantitative description of the motional geometry of the carboranes several stochastic models are formulated. By comparison of the model calculations with the experimental results it is found that the dynamics of these quasi-icosahedrally shaped molecules is governed by a composite reorientation process. Here the molecules perform threefold jumps around a molecule-fixed axis which itself can be tilted in four different directions with respect to a crystal-fixed axis. The tilt angle increases significantly with increasing temperature. On the basis of measurements of four-time stimulated-echo functions, implications for dynamic heterogeneity also in comparison with that of supercooled liquids are discussed