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Using ab initio molecular dynamics simulations and nudged elastic band calculations we examine the finite temperature stability, transition pathways, and migration mechanisms of large oxygen clusters in UO2+x. Here we specifically consider the recently proposed split quad-interstitial and cuboctahedral oxygen clusters. It is shown that isolated cuboctahedral clusters may transform into more stable configurations that are closely linked to the split quad-interstitial. The split quad-interstitial is stable with respect to single interstitials occupying the empty octahedral holes of the UO2 lattice. In order to better understand discrepancies between theory and experiments, the simulated atomic pair distribution functions for the split quad-interstitial structures are analyzed with respect to the distribution function for U4O9 previously obtained from neutron diffraction data. Our nudged elastic band calculations suggest that the split quad-interstitial may migrate by translating one of its constituent di-interstitial clusters via a barrier that is lower than the corresponding barrier for individual interstitials, but higher than the barrier for the most stable di-interstitial cluster. [ABSTRACT FROM AUTHOR]

A quasielastic neutron scattering study has been performed on a polymer gel electrolyte consisting of lithium perchlorate dissolved in ethylene carbonate/propylene carbonate and stabilized with poly(methyl methacrylate). The dynamics of the solvent, which is crucial for the ion conduction in this system, was probed using the hydrogen/deuterium contrast variation method with nondeuterated solvent and a deuterated polymer matrix. Two relaxation processes of the solvent were studied in the 10–400 μeV range at different temperatures. From analysis of the momentum transfer dependence of the processes we conclude that the faster process (∼100 μeV) is related to rotational diffusion of the solvent and the slower process (∼10 μeV) to translational diffusion of the solvent. The translational diffusion is found to be similar to the diffusion in the corresponding liquid electrolyte at short distances, but geometrically constrained by the polymer matrix at distances beyond ∼5 Å. The study indicates that the hindered diffusion of the solvent on a length scale of the polymer network interchain distance (∼5–20 Å) is sufficient to explain the reduced macroscopic diffusivity and ion conductivity of the gel electrolyte compared to the liquid electrolyte. [ABSTRACT FROM AUTHOR]

The structure of an amorphous polymer electrolyte, poly(propylene oxide) (PPO) complexed with LiClO4, has been studied using reverse Monte Carlo (RMC) simulations. The simulations require no force field but are based on experimental data only, in this case from x-ray and neutron diffraction experiments. Excellent agreement between the experimental data and the structures resulting from the RMC simulation is obtained. Samples with ether-oxygen to lithium concentrations (molar ratios) O:Li=16:1 and 5:1 were studied and compared to results of pure PPO from a previous study. We focus on the effects of the solvated salt on the structure of the polymer matrix, the spatial distribution of ions, and the correlations between the anions and the polymer chains. Analyzing the structures produced in the simulations, we find that for a concentration 16:1, the interchain distance is approximately the same as in pure PPO but more well defined. For a concentration 5:1, we find a larger and less well-defined interchain distance compared to the 16:1 concentration. This signifies that at the 16:1 salt concentration, there is enough free volume in the polymer host to accommodate the ions, and that the solvation of salt induces ordering of the polymer matrix. At the higher salt concentration 5:1, the polymer network must expand and become less ordered to host the ions. We also note, in accordance with previous studies, that the solvation of salt changes the conformation of the polymer chain towards more gauche states. The simulations furthermore reveal marked correlations between the polymer chains and the anions, which we suggest arise predominantly from an interaction mediated via cations, which can simultaneously coordinate both ether oxygens in the polymer chains and anions. Interanionic distances at 5 Å, which are consistent with two or more anions being coordinated around the same cation, are also observed. On a larger scale, the RMC structure of PPO-LiClO4 16:1 clearly indicates the presence of salt-rich and salt-depleted domains having a length scale of <20 Å. In view of such a heterogeneous structure of PPO-LiClO4 16:1, it is plausible that the increased ordering of the polymer matrix is due to rather well-defined structural arrangements within the salt-rich domains, and that the characteristic interchain distance in the salt-rich domains is similar to that of the pure polymer.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structural dynamics of a polymer electrolyte model material, poly(prolyene oxide) (PPO)-LiClO[sub 4] (and PPO for reference), has for the first time been studied using coherent quasielastic neutron scattering. By a combination of neutron spin echo and inverse time-of-flight techniques we investigate the relaxation function in an experimental time window 10[sup -12]≤t≤10[sup -8] s at a momentum transfer corresponding to the distance between neighboring interchain segments. We find that the relaxation of the correlation between neighboring chains is slower and more stretched in the polymer salt complex compared to the pure polymer. The data can, for both PPO and PPO-LiClO[sub 4], be described by a stretched exponential function with temperature independent stretching parameters. While the relaxation times follow the macroscopic viscosity for the former, they do not for the latter. The slower relaxation in PPO-LiClO[sub 4] compared to PPO and the failure of the viscosity scaling in PPO-LiClO[sub 4] may be explained in terms of a temperature dependent effective molecular weight induced by cations acting as cross links between chains. We discuss the origin of the extra stretching of the relaxation in the polymer salt complex under the aspect of heterogeneity, comparing it with data in the literature. We find that the stretching to the major part is intrinsic or at most due to heterogeneities on an atomic length scale. The molecular length scale of the experiment allows for the first time a direct connection to the renewal time in the dynamic disordered hopping model for ion transport in polymer electrolytes. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]