On the Discrepancy between Local and Average Structure in the Fast Na+ Ionic Conductor Na2.9Sb0.9W0.1S4.

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS43- by WS42- in Na2.9Sb0.9W0.1S4 leads to a very high ionic conductivity of 41 mS cm-1 at room temperature. While pristine Na3SbS4 crystallizes in a tetragonal structure, the substituted Na2.9Sb0.9W0.1S4 crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse 121Sb NMR experiments that the short-range order of Na2.9Sb0.9W0.1S4 remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na+ substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na3SbS4, quasi-elastic neutron scattering and solid-state 23Na nuclear magnetic resonance measurements revealed drastically improved Na+ diffusivity and decreased activation energies for Na2.9Sb0.9W0.1S4. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.

Uncovering the hydride ion diffusion pathway in barium hydride via neutron spectroscopy.

Solid state materials possessing the ability for fast ionic diffusion of hydrogen have immense appeal for a wide range of energy-related applications. Ionic hydrogen transport research is dominated by proton conductors, but recently a few examples of hydride ion conductors have been observed as well. Barium hydride, BaH2, undergoes a structural phase transition around 775 K that leads to an order of magnitude increase in the ionic conductivity. This material provides a prototypical system to understand hydride ion diffusion and how the altered structure produced by the phase transition can have an enormous impact on the diffusion. We employ quasielastic and inelastic neutron scattering to probe the atomic scale diffusion mechanism and vibrational dynamics of hydride ions in both the low- and high-temperature phases. Jump lengths, residence times, diffusion coefficients, and activation energies are extracted and compared to the crystal structure to uncover the diffusion pathways. We find that the hydrogen jump distances, residence times, and energy barriers become reduced following the phase transition, allowing for the efficient conduction of hydride ions through a series of hydrogen jumps of length L = 3.1 Å.

Correlating Broadband Photoluminescence with Structural Dynamics in Layered Hybrid Halide Perovskites.

The emission of white light from a single material is atypical and is of interest for solid-state lighting applications. Broadband light emission has been observed in some layered perovskite derivatives, A2PbBr4 (A = R-NH3+), and correlates with static structural distortions corresponding to out-of-plane tilting of the lead bromide octahedra. While materials with different organic cations can yield distinct out-of-plane tilts, the underlying origin of the octahedral tilting remains poorly understood. Using high energy resolution (e.g., quasi-elastic) neutron scattering, this contribution details the rotational dynamics of the organic cations in A2PbBr4 materials where A = n-butylammonium (nBA), 1,8-diaminooctammonium (ODA), and 4-aminobutyric acid (GABA). The organic cation dynamics differentiate (nBA)2PbBr4 from (ODA)PbBr4 or (GABA)2PbBr4 in that the larger spatial extent of dynamics of nBA yields a larger effective cation radius. The larger effective volume of the nBA cation in (nBA)2PbBr4 yields a closer to ideal A-site geometry, preventing the out-of-plane tilt and broadband luminescence. In all three compounds, we observe hydrogen dynamics attributed to rotation of the ammonium headgroup and at a time scale faster than the white light photoluminescence studied by time-correlated single photon counting spectroscopy. This supports a previous assignment of the broadband emission as resulting from a single ensemble, such that the emissive excited state experiences many local structures faster than the emissive decay. The findings presented here highlight the role of the organic cation and its dynamics in hybrid organic-inorganic perovskites and white light emission.

Temperature and salt controlled tuning of protein clusters.

The formation of molecular assemblies in protein solutions is of strong interest both from a fundamental viewpoint and for biomedical applications. While ordered and desired protein assemblies are indispensable for some biological functions, undesired protein condensation can induce serious diseases. As a common cofactor, the presence of salt ions is essential for some biological processes involving proteins, and in aqueous suspensions of proteins can also give rise to complex phase diagrams including homogeneous solutions, large aggregates, and dissolution regimes. Here, we systematically study the cluster formation approaching the phase separation in aqueous solutions of the globular protein BSA as a function of temperature (T), the protein concentration (cp) and the concentrations of the trivalent salts YCl3 and LaCl3 (cs). As an important complement to structural, i.e. time-averaged, techniques we employ a dynamical technique that can detect clusters even when they are transient on the order of a few nanoseconds. By employing incoherent neutron spectroscopy, we unambiguously determine the short-time self-diffusion of the protein clusters depending on cp, cs and T. We determine the cluster size in terms of effective hydrodynamic radii as manifested by the cluster center-of-mass diffusion coefficients D. For both salts, we find a simple functional form D(cp, cs, T) in the parameter range explored. The calculated inter-particle attraction strength, determined from the microscopic and short-time diffusive properties of the samples, increases with salt concentration and temperature in the regime investigated and can be linked to the macroscopic behavior of the samples.

Bromine Incorporation and Suppressed Cation Rotation in Mixed-Halide Perovskites.

Engineering the composition of perovskite active layers has been critical in increasing the efficiency of perovskite solar cells (PSCs) to more than 25% in the latest reports. Partial substitutions of the monovalent cation and the halogen have been adopted in the highest-performing devices, but the precise role of bromine incorporation remains incompletely explained. Here we use quasi-elastic neutron scattering (QENS) to study, as a function of the degree of bromine incorporation, the dynamics of organic cations in triple-cation lead mixed-halide perovskites. We find that the inclusion of bromine suppresses low-energy rotations of formamidinium (FA), and we find that inhibiting FA rotation correlates with a longer-lived carrier lifetime. When the fraction of bromine approaches 0.15 on the halogen site-a composition used extensively in the PSC literature-the fraction of actively rotating FA molecules is minimized: indeed, the fraction of rotating FA is suppressed by more than 25% compared to the bromine-free perovskite.

Strong Adverse Contribution of Conformational Dynamics to Streptavidin-Biotin Binding.

Molecular dynamics plays an important role for the biological function of proteins. For protein ligand interactions, changes of conformational entropy of protein and hydration layer are relevant for the binding process. Quasielastic neutron scattering (QENS) was used to investigate differences in protein dynamics and conformational entropy of ligand-bound and ligand-free streptavidin. Protein dynamics were probed both on the fast picosecond time scale using neutron time-of-flight spectroscopy and on the slower nanosecond time scale using high-resolution neutron backscattering spectroscopy. We found the internal equilibrium motions of streptavidin and the corresponding mean square displacements (MSDs) to be greatly reduced upon biotin binding. On the basis of the observed MSDs, we calculated the difference of conformational entropy ΔSconf of the protein component between ligand-bound and ligand-free streptavidin. The rather large negative ΔSconf value (-2 kJ mol-1 K-1 on the nanosecond time scale) obtained for the streptavidin tetramer seems to be counterintuitive, given the exceptionally high affinity of streptavidin-biotin binding. Literature data on the total entropy change ΔS observed upon biotin binding to streptavidin, which includes contributions from both the protein and the hydration water, suggest partial compensation of the unfavorable ΔSconf by a large positive entropy gain of the surrounding hydration layer and water molecules that are displaced during ligand binding.

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 encapsulated hepatitis B surface antigen: a combined neutron spectroscopy and thermo-analysis study

As a consequence of its ordered pore architecture, mesoporous SBA-15 offers new possibilities for incorporating biological agents. Considering its applicability in oral vaccination, which shows more beneficial features when compared with parenteral vaccines, SBA-15 is also seen as a very promising adjuvant to carry, protect, and deliver entrapped antigens. Recent studies have shown several remarkable features in the immunization of hepatitis B, a viral disease transmitted mainly through blood or serum transfer. However, the surface antigen of the hepatitis B virus, HBsAg, is too large to fit inside the SBA-15 matrix with mean pore diameter around 10 nm, thus raising the question of how SBA-15 can protect the antigen. In this work, thermal analysis combined with neutron spectroscopy allowed us to shed light on the interactions between HBsAg and SBA-15 as well as on the role that these interactions play in the efficiency of this promising oral vaccination method. This information was obtained by verifying how the dynamic behaviour of the antigen is modified under confinement in SBA-15, thus also establishing an experimental method for verifying molecular dynamics simulations.

Dynamics of encapsulated hepatitis B surface antigen: a combined neutron spectroscopy and thermo-analysis study

As a consequence of its ordered pore architecture, mesoporous SBA-15 offers new possibilities for incorporating biological agents. Considering its applicability in oral vaccination, which shows more beneficial features when compared with parenteral vaccines, SBA-15 is also seen as a very promising adjuvant to carry, protect, and deliver entrapped antigens. Recent studies have shown several remarkable features in the immunization of hepatitis B, a viral disease transmitted mainly through blood or serum transfer. However, the surface antigen of the hepatitis B virus, HBsAg, is too large to fit inside the SBA-15 matrix with mean pore diameter around 10 nm, thus raising the question of how SBA-15 can protect the antigen. In this work, thermal analysis combined with neutron spectroscopy allowed us to shed light on the interactions between HBsAg and SBA-15 as well as on the role that these interactions play in the efficiency of this promising oral vaccination method. This information was obtained by verifying how the dynamic behaviour of the antigen is modified under confinement in SBA-15, thus also establishing an experimental method for verifying molecular dynamics simulations.

Fractal diffusion in high temperature polymer electrolyte fuel cell membranes.

The performance of fuel cells depends largely on the proton diffusion in the proton conducting membrane, the core of a fuel cell. High temperature polymer electrolyte fuel cells are based on a polymer membrane swollen with phosphoric acid as the electrolyte, where proton conduction takes place. We studied the proton diffusion in such membranes with neutron scattering techniques which are especially sensitive to the proton contribution. Time of flight spectroscopy and backscattering spectroscopy have been combined to cover a broad dynamic range. In order to selectively observe the diffusion of protons potentially contributing to the ion conductivity, two samples were prepared, where in one of the samples the phosphoric acid was used with hydrogen replaced by deuterium. The scattering data from the two samples were subtracted in a suitable way after measurement. Thereby subdiffusive behavior of the proton diffusion has been observed and interpreted in terms of a model of fractal diffusion. For this purpose, a scattering function for fractal diffusion has been developed. The fractal diffusion dimension dw and the Hausdorff dimension df have been determined on the length scales covered in the neutron scattering experiments.

Dynamics of Water Associated with Lithium Ions Distributed in Polyethylene Oxide.

The dynamics of water in polyethylene oxide (PEO)/LiCl solution has been studied with quasielastic neutron scattering experiments and molecular dynamics (MD) simulations. Two different time scales of water diffusion representing interfacial water and bulk water dynamics have been identified. The measured diffusion coefficient of interfacial water remained 5-10 times smaller than that of bulk water, but both were slowed by approximately 50% in the presence of Li(+). Detailed analysis of MD trajectories suggests that Li(+) is favorably found at the surface of the hydration layer, and the probability to find the caged Li(+) configuration formed by the PEO is lower than for the noncaged Li(+)-PEO configuration. In both configurations, however, the slowing down of water molecules is driven by reorienting water molecules and creating water-Li(+) hydration complexes. Performing the MD simulation with different ions (Na(+) and K(+)) revealed that smaller ionic radius of the ions is a key factor in disrupting the formation of PEO cages by allowing spaces for water molecules to come in between the ion and PEO.

Salt-Induced Universal Slowing Down of the Short-Time Self-Diffusion of a Globular Protein in Aqueous Solution.

The short-time self-diffusion D of the globular model protein bovine serum albumin in aqueous (D2O) solutions has been measured comprehensively as a function of the protein and trivalent salt (YCl3) concentration, noted cp and cs, respectively. We observe that D follows a universal master curve D(cs,cp) = D(cs = 0,cp) g(cs/cp), where D(cs = 0,cp) is the diffusion coefficient in the absence of salt and g(cs/cp) is a scalar function solely depending on the ratio of the salt and protein concentration. This observation is consistent with a universal scaling of the bonding probability in a picture of cluster formation of patchy particles. The finding corroborates the predictive power of the description of proteins as colloids with distinct attractive ion-activated surface patches.

Hierarchical molecular dynamics of bovine serum albumin in concentrated aqueous solution below and above thermal denaturation.

The dynamics of proteins in solution is a complex and hierarchical process, affected by the aqueous environment as well as temperature. We present a comprehensive study on nanosecond time and nanometer length scales below, at, and above the denaturation temperature Td. Our experimental data evidence dynamical processes in protein solutions on three distinct time scales. We suggest a consistent physical picture of hierarchical protein dynamics: (i) self-diffusion of the entire protein molecule is confirmed to agree with colloid theory for all temperatures where the protein is in its native conformational state. At higher temperatures T > Td, the self-diffusion is strongly obstructed by cross-linking or entanglement. (ii) The amplitude of backbone fluctuations grows with increasing T, and a transition in its dynamics is observed above Td. (iii) The number of mobile side-chains increases sharply at Td while their average dynamics exhibits only little variations. The combination of quasi-elastic neutron scattering and the presented analytical framework provides a detailed microscopic picture of the protein molecular dynamics in solution, thereby reflecting the changes of macroscopic properties such as cluster formation and gelation.

Elasticity and Inverse Temperature Transition in Elastin.

Elastin is a structural protein and biomaterial that provides elasticity and resilience to a range of tissues. This work provides insights into the elastic properties of elastin and its peculiar inverse temperature transition (ITT). These features are dependent on hydration of elastin and are driven by a similar mechanism of hydrophobic collapse to an entropically favorable state. Using neutron scattering, we quantify the changes in the geometry of molecular motions above and below the transition temperature, showing a reduction in the displacement of water-induced motions upon hydrophobic collapse at the ITT. We also measured the collective vibrations of elastin gels as a function of elongation, revealing no changes in the spectral features associated with local rigidity and secondary structure, in agreement with the entropic origin of elasticity.

Direct measurement of hydrogen dislocation pipe diffusion in deformed polycrystalline Pd using quasielastic neutron scattering.

The temperature-dependent diffusivity D(T) of hydrogen solute atoms trapped at dislocations-dislocation pipe diffusion of hydrogen-in deformed polycrystalline PdH(x) (x∼10(-3) [H]/[Pd]) has been quantified with quasielastic neutron scattering between 150 and 400 K. We observe diffusion coefficients for trapped hydrogen elevated by one to two orders of magnitude above bulk diffusion. Arrhenius diffusion behavior has been observed for dislocation pipe diffusion and regular bulk diffusion, the latter in well-annealed polycrystalline Pd. For regular bulk diffusion of hydrogen in Pd we find D(T)=D(0)exp(-E(a)/kT)=0.005exp(-0.23 eV/kT) cm(2)/s, in agreement with the known diffusivity of hydrogen in Pd. For hydrogen dislocation pipe diffusion we find D(T)≃10(-5)exp(-E(a)/kT) cm(2)/s, where E(a)=0.042 and 0.083 eV for concentrations of 0.52×10(-3) and 1.13×10(-3)[H]/[Pd], respectively. Ab initio computations provide a physical basis for the pipe diffusion pathway and confirm the reduced barrier height.

Low energy nuclear spin excitations in Ho metal investigated by high resolution neutron spectroscopy.

We have investigated the low energy excitations in metallic Ho by high resolution neutron spectroscopy. We found at T = 3 K clear inelastic peaks in the energy loss and energy gain sides, along with the central elastic peak. The energy of this low energy excitation, which is 26.59 ± 0.02 µeV at T = 3 K, decreased continuously and became zero at TN ≈ 130 K. By fitting the data in the temperature range 100-127.5 K with a power law we obtained the power-law exponent ß = 0.37 ± 0.02, which agrees with the expected value ß = 0.367 for a three-dimensional Heisenberg model. Thus the energy of the low energy excitations can be associated with the order parameter.

Dynamics of Water in NaxCoO2.yH2O.

Incoherent inelastic neutron scattering experiments were performed on Na0.7CoO2 and Na0.28CoO2.1.3H2O in order to understand how the dynamics of the hydrogen-bond network of water is modified in the triangular crystalline lattice NaxCoO2.yH2O. 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 Na0.28CoO2.1.3H2O, 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 x 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.

The dynamical properties of the aromatic hydrogen bond in NH4(C6H5)4B from quasielastic neutron scattering.

NH(4)(C(6)H(5))(4)B represents a prototypical system for understanding aromatic H bonds. In NH(4)(C(6)H(5))(4)B an ammonium cation is trapped in an aromatic cage of four phenyl rings and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion as donor. Here the dynamical properties of the aromatic hydrogen bond in NH(4)(C(6)H(5))(4)B were studied by quasielastic incoherent neutron scattering in a broad temperature range (20< or =T< or =350 K). We show that in the temperature range from 67 to 350 K the ammonium ions perform rotational jumps around C(3) axes. The correlation time for this motion is the lifetime of the "transient" H bonds. It varies from 1.5 ps at T=350 K to 150 ps at T=67 K. The activation energy was found to be 3.14 kJ mol, which means only 1.05 kJ mol per single H bond for reorientations around the C(3) symmetry axis of the ammonium group. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate and thereby break their H bonds. In addition, at temperatures above 200 K local diffusive reorientational motions of the phenyl rings, probably caused by interaction with ammonium-group reorientations, were found within the experimental observation time window. At room temperature a reorientation angle of 8.4 degrees +/-2 degrees and a correlation time of 22+/-8 ps were determined for the latter. The aromatic H bonds are extremely short lived due to the low potential barriers allowing for molecular motions with a reorientational character of the donors. The alternating rupture and formation of H bonds causes very strong damping of the librational motion of the acceptors, making the transient H bond appear rather flexible.