Disorder in La1-xBa1+xGaO4-x/2 ionic conductor: resolving Pair Distribution Function through inside from first principles modeling

Ionic conduction in dry LaBaGaO4 occurs through the vacant oxygen sites formed by the substitution of Ba for La. The resulting La1 12xBa1+xGaO4 12x/2 solid solution shows significant disorder characteristics. The local structure of compositions x = 0, 0.20 and 0.30 was studied using the pair distribution function (PDF). Unfortunately, increasing peak overlap and the number of independent structural parameters make PDF modeling challenging when dealing with low-symmetry phases. To overcome this problem, density functional theory (DFT) was employed to create different structural models, each one with a different relative position for the substitutional Ba ion with respect to the oxygen vacancy. The atomic distributions generated by DFT were used as a starting point to refine experimental PDF data. All models result in the formation of Ga2O7 dimers, with their major axis oriented along the c axis. At the local scale, the most stable DFT model also provides the best fit of the PDF. This accounts for the dopant as first and second neighbors of the vacancy and of the O bridge in the dimer, suggesting that substitutional barium ions act as pinning centers for oxygen vacancies. Above 6 \uc5 the average orthorhombic structure fits the PDF better than the DFT models, thus indicating that Ga2O7 dimers are not correlated with each other to form extended ordered structures. The combination of DFT simulations and X-ray diffraction/PDF refinements was used successfully to model the local atomic structure in La1 12xBa1+xGaO4 12x/2, thus suggesting that this approach could be positively applied in general to disordered systems

Excess wing in glass-forming glycerol and LiCl-glycerol mixtures detected by neutron scattering

The relaxational dynamics in glass-forming glycerol and glycerol mixed with LiCl is in-vestigated using different neutron scattering techniques. The performed neutron spin-echo experiments, which extend up to relatively long relaxation-time scales of the order of 10 ns, should allow for the detection of contributions from the so-called excess wing. This phenomenon, whose microscopic origin is controversially discussed, arises in a variety of glass formers and, until now, was almost exclusively investigated by dielectric spectros-copy and light scattering. Here we show that the relaxational process causing the excess wing also can be detected by neutron scattering, which directly couples to density fluctua-tions.Comment: 8 pages, 6 figure

Effect of adding nanometre-sized heterogeneities on the structural dynamics and the excess wing of a molecular glass former

We present the relaxation dynamics of glass-forming glycerol mixed with 1.1 nm sized polyhedral oligomeric silsesquioxane (POSS) molecules using dielectric spectroscopy (DS) and two different neutron scattering (NS) techniques. Both, the reorientational dynamics as measured by DS and the density fluctuations detected by NS reveal a broadening of the alpha relaxation when POSS molecules are added. Moreover, we find a significant slowing down of the alpha-relaxation time. These effects are in accord with the heterogeneity scenario considered for the dynamics of glasses and supercooled liquids. The addition of POSS also affects the excess wing in glycerol arising from a secondary relaxation process, which seems to exhibit a dramatic increase in relative strength compared to the alpha-relaxation.Comment: 32 pages, 7 figures, accepted for publication in the journal Scientific Report

Effects of water on the stochastic motions of propane confined in MCM-41-S pores

Hydrocarbons confined in porous media find applications in a wide variety of industries and therefore their diffusive behavior is widely studied. Most of the porous media found in natural environments are laden with water, which might affect the confined hydrocarbons. To quantify the effect of hydration, we report here a combined quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulation study on the dynamics of propane confined in the 1.5 nm-wide micropores of MCM-41-S in the presence of water at 230 and 250 K. To eliminate the strong incoherent signal from water and emphasize the propane signal we have used heavy water (D2O). QENS data show two dynamically different populations of propane in MCM-41-S and suggest that the presence of water hinders the diffusion of propane. Weak elastic contributions to the QENS spectra suggest that only long-range translational motion of propane molecules contributes to the quasielastic broadening. MD simulations carried out using a model cylindrical silica pore of 1.6 nm diameter filled with water and propane agree with the experimental finding of water hindering the diffusion of propane. Further, the simulation results suggest that the slowing down of propane motions is a function of the water content within the pore and is stronger at higher water contents. At high water content, the structure and the dynamics, both translational and rotational, of propane are severely impacted. Simulation data suggest that the rotational motion of the propane molecule occurs on time scales much faster than those accessible with the QENS instrument used, and thus explain the weak elastic contribution to the QENS spectra measured in the experiments. This study shows the effects of hydration on the structure and dynamics of volatiles in porous media, which are of interest for fundamental understanding and applied studies of confined fluids

Dynamics of water in NaxCoO2 yH2O

Incoherent inelastic neutron scattering experiments were performed on Na 0.7 CoO2 and Na0.28 CoO2 1.3H2 O in order to understand how the dynamics of the hydrogen bond network of water is modified in the triangular crystalline lattice Na x CoO2 yH2 O. Using quasi elastic neutron scattering QENS we were able to differentiate between two types of proton dynamics a fast process due to water strongly bound into the sodium cobalt oxyhydrate structure during the hydration process and a slow process likely attributable to a collective motion . High resolution QENS experiments, carried out on Na0.28CoO2 1.3H2 O, show that at temperatures above 310 K the water dynamics can be well described by a random jump diffusion model characterized by a diffusion constant equal to 0.9.10?9 m2 s, which is significantly lower than the rate of diffusion for bulk water. Furthermore, our results indicate that at room temperature the sodium ions have no influence on the rotational dynamics of the fast water molecules

Effects of configurational changes on molecular dynamics in polyvinylidene fluoride and poly(vinylidene fluoride-trifluoroethylene) ferroelectric polymers

We present a comparative study of proton dynamics in unpoled non-ferroelectric polymer polyvinylidene fluoride (PVDF) and in its trifluoroethylene containing ferroelectric copolymer (with 70/30 molar proportion), using quasi-elastic neutron scattering. The neutron data reveal the existence of two distinct types of molecular motions in the temperature range investigated. The slower motion, which is characterized in details here, is ascribed to protons jump diffusion along the polymeric carbon chains, while the faster motion could be attributed to localized rotational motion of methylene groups. At temperatures below the Curie point (Tc  ∼ 385 K) of the composite polymer, the slower diffusive mode experiences longer relaxation times in the ferroelectric blend than in the bare PVDF, although the net corresponding diffusion coefficient remains comparatively the same in both polymers with characteristic activation energy of EA  ≈ 27–33 kJ/mol. This arises because of a temperature dependent jump length r 0, which we observe to be effectively longer in the copolymer, possibly due to the formation of ordered ferroelectric domains below Tc . Above Tc , there is no appreciable difference in r 0 between the two systems. This observation directly relates the known dependence of Tc on molar ratio to changes in r 0, providing fundamental insight into the ferroelectric properties of PVDF-based copolymers

Direct observation of low energy nuclear spin excitations in HoCrO 3 by high resolution neutron spectroscopy

We have investigated low energy nuclear spin excitations in the strongly correlated electron compound HoCrO3. Weobserve clear inelastic peaks at E D 22:18 0:04 eV in both energy loss and gain sides. The energy of the inelasticpeaks remains constant in the temperature range 1.5–40 K at which they are observed. The intensity of the inelasticpeak increases at first with increasing temperature and then decreases at higher temperatures. The temperaturedependence of the energy and intensity of the inelastic peaks is very unusual compared to that observed in other Nd, Co,V and also simple Ho compounds. Huge quasielastic scattering appears at higher temperatures presumably due to thefluctuating electronic moments of the Ho ions that get increasingly disordered at higher temperatures. The strongquasielastic scattering may also originate in the first Ho crystal-field excitations at about 1.5 meV

Rotational motion of the water molecules in the superconductor Na0.28CoO2 dot yH2O

Abstract. Quasielastic neutron scattering QENS has been used to investigate the dynamical behaviour of H2O in the superconductor Na0.28CoO2.yH2O. The measurements were obtained at room temperature for a molar concentration of water, y 1.3. From the analysis of the Q dependence of the EISF and the line width we found that the water dynamics can be well described by jump rotations that take the molecule into identical orientation. We were also able to define a rotational correlation time of amp; 61556; 1.85p