Backward correlations and dynamic heterogeneities: a computer study of ion dynamics

We analyse the correlated back and forth dynamics and dynamic heterogeneities, i.e. the presence of fast and slow ions, for a lithium metasilicate system via computer simulations. For this purpose we define, in analogy to previous work in the field of glass transition, appropriate three-time correlation functions. They contain information about the dynamics during two successive time intervals. First we apply them to simple model systems in order to clarify their information content. Afterwards we use this formalism to analyse the lithium trajectories. A strong back-dragging effect is observed, which also fulfills the time-temperature superposition principle. Furthermore, it turns out that the back-dragging effect is long-ranged and exceeds the nearest neighbor position. In contrast, the strength of the dynamic heterogeneities does not fulfill the time-temperature superposition principle. The lower the temperature, the stronger the mobility difference between fast and slow ions. The results are then compared with the simple model systems considered here as well as with some lattice models of ion dynamics.Comment: 12 pages, 10 figure

Suppression of thermal conductivity without impeding electron mobility in n-type XNiSn half-Heusler thermoelectrics

We outline a strategy to improve the thermoelectric performance of n-type XNiSn based half-Heusler alloys through Cu doping into vacant tetrahedral sites. A comprehensive combination of structural characterisation and modelling is employed to discriminate the competing mechanisms for thermoelectric enhancement. During synthesis a mineralising effect occurs that improves the homogeneity of the alloying elements Ti, Zr and Hf, and promotes grain growth, leading to a doubling of the electron mobility. In the formed materials, Cu is a strong n-type dopant, like Sb, but occupies the interstitial site and strongly enhances phonon scattering without diminishing carrier mobility (in contrast to interstitial Ni). Simultaneous alloying with Ti, Zr and Hf serves to minimise the thermal conductivity via regular mass disorder and strain effects. A best electronic power factor, S2/ρ, of 3.6 mW m−1 K−2 and maximum ZT of 0.8 at 773 K were observed for a Ti0.5Zr0.25Hf0.25NiCu0.025Sn composition, enabling promising device power densities of ∼6 W cm−2 and ∼8% conversion efficiency from a 450 K gradient. These findings are important because they provide new insight into the mechanisms underpinning high ZT in the XNiSn system and indicate a direction for further improvements in thermoelectric performance

Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions

Molecular dynamics simulations are performed to study the lithium jumps in LiPO3 glass. In particular, we calculate higher-order correlation functions that probe the positions of single lithium ions at several times. Three-time correlation functions show that the non-exponential relaxation of the lithium ions results from both correlated back-and-forth jumps and the existence of dynamical heterogeneities, i.e., the presence of a broad distribution of jump rates. A quantitative analysis yields that the contribution of the dynamical heterogeneities to the non-exponential depopulation of the lithium sites increases upon cooling. Further, correlated back-and-forth jumps between neighboring sites are observed for the fast ions of the distribution, but not for the slow ions and, hence, the back-jump probability depends on the dynamical state. Four-time correlation functions indicate that an exchange between fast and slow ions takes place on the timescale of the jumps themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites featuring fast and slow lithium dynamics, respectively, are intimately mixed. In addition, a backward correlation beyond the first neighbor shell for highly mobile ions and the presence of long-range dynamical heterogeneities suggest that fast ion migration occurs along preferential pathways in the glassy matrix. In the melt, we find no evidence for correlated back-and-forth motions and dynamical heterogeneities on the length scale of the next-neighbor distance.Comment: 12 pages, 13 figure

Water Chemisorption and Reconstruction of the MgO Surface

The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with Delta E = -90.2kJ/mol. We conclude that protonation stabilizes the otherwise unstable (111) surface and that this, not the bare (001), is the most stable surface of MgO under ambient conditions.Comment: RevTeX, 4 pages, 1 Encapsulated Postscript Figur

Supramolecular binding and separation of hydrocarbons within a functionalised porous metal-organic framework

Supramolecular interactions are fundamental to host-guest binding in chemical and biological processes. Direct visualisation of such supramolecular interactions within host-guest systems is extremely challenging but crucial for the understanding of their function. We report a comprehensive study combining neutron scattering with synchrotron X-ray and neutron diffraction, coupled with computational modelling, to define the detailed binding at a molecular level of acetylene, ethylene and ethane within the porous host NOTT-300. This study reveals the simultaneous and cooperative hydrogen-bonding, π···π stacking interactions and inter-molecular dipole interactions in the binding of acetylene and ethylene to give up to twelve individual weak supramolecular interactions aligned within the host to form an optimal geometry for intelligent, selective binding of hydrocarbons. We also report, for the first time, the cooperative binding of a mixture of acetylene and ethylene within the porous host together with the corresponding breakthrough experiment and analysis of mixed gas adsorption isotherms

Animated molecular dynamics simulations of hydrated caesium-smectite interlayers

Computer animation of center of mass coordinates obtained from 800 ps molecular dynamics simulations of Cs-smectite hydrates (1/3 and 2/3 water monolayers) provided information concerning the structure and dynamics of the interlayer region that could not be obtained through traditional simulation analysis methods. Cs(+ )formed inner sphere complexes with the mineral surface, and could be seen to jump from one attracting location near a layer charge site to the next, while water molecules were observed to migrate from the hydration shell of one ion to that of another. Neighboring ions maintained a partial hydration shell by sharing water molecules, such that a single water molecule hydrated two ions simultaneously for hundreds of picoseconds. Cs-montmorillonite hydrates featured the largest extent of this sharing interaction, because interlayer ions were able to inhabit positions near surface cavities as well as at their edges, close to oxygen triads. The greater positional freedom of Cs(+ )within the montmorillonite interlayer, a result of structural hydroxyl orientation and low tetrahedral charge, promoted the optimization of distances between cations and water molecules required for water sharing. Preference of Cs(+ )for locations near oxygen triads was observed within interlayer beidellite and hectorite. Water molecules also could be seen to interact directly with the mineral surface, entering its surface cavities to approach attracting charge sites and structural hydroxyls. With increasing water content, water molecules exhibited increased frequency and duration of both cavity habitation and water sharing interactions. Competition between Cs(+ )and water molecules for surface sites was evident. These important cooperative and competitive features of interlayer molecular behavior were uniquely revealed by animation of an otherwise highly complex simulation output