Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6.

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO's quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (⊥ b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.

Ni self-diffusion in glass forming Pd-Ni-S melts.

The Ni self-diffusion in glass forming Pd40Ni40S20, Pd37Ni37S26and Pd31Ni42S27melts was probed by incoherent, quasielastic neutron scattering over a temperature range between 773 and 1023 K. The Ni self-diffusion coefficients are on a 10-10 m2 s-1-10-9 m2 s-1scale and barely change with composition. Each composition exhibits an Arrhenius-type temperature dependence of the Ni self-diffusion coefficients, which results in activation energies ranging fromEA= 348 ± 16 meV for Pd40Ni40S20toEA= 387 ± 6 meV for Pd37Ni37S26. The structural relaxation shows a stretched exponential behavior even far above the liquidus temperatures. In addition, the viscosity of the Pd37Ni37S26melt was measured under reduced gravity conditions. The diffusion calculated from the viscosity reveals a significant deviation from the measured Ni self-diffusion by a factor between 4 and 8. This may indicate a dynamic decoupling between the atoms within the Pd-Ni-S equilibrium melts.

Elucidation of Diffusivity of Hydrogen Isotopes in Flexible MOFs by Quasi-Elastic Neutron Scattering.

Kinetic-quantum-sieving-assisted H2 :D2 separation in flexible porous materials is more effective than the currently used energy-intensive cryogenic distillation and girdle-sulfide processes for isotope separation. It is believed that material flexibility results in a pore-breathing phenomenon under the influence of external stimuli, which helps in adjusting the pore size and gives rise to the optimum quantum-sieving phenomenon at each stage of gas separation. However, only a few studies have investigated kinetic-quantum-sieving-assisted isotope separation using flexible porous materials. In addition, no reports are available on the microscopic observation of isotopic molecular transportation during the separation process under dynamic transition. Here, the experimental observation of a significantly faster diffusion of deuterium than hydrogen in a flexible pore structure, even at high temperatures, through quasi-elastic neutron scattering, is reported. Unlike rigid structures, the extracted diffusion dynamics of hydrogen isotopes within flexible frameworks show that the diffusion difference between the isotopes increases with an increase in temperature. Owing to this unique inverse trend, a new strategy is suggested for achieving higher operating temperatures for efficient isotope separation utilizing a flexible metal-organic framework system.

Dynamics in hydrated inorganic nanotubes studied by neutron scattering: towards nanoreactors in water.

Water dynamics in inorganic nanotubes is studied by neutron scattering technique. Two types of aluminosilicate nanotubes are investigated: one is completely hydrophilic on the external and internal surfaces (IMO-OH) while the second possesses an internal cavity which is hydrophobic due to the replacement of Si-OH bonds by Si-CH3 ones (IMO-CH3), the external surface being still hydrophilic. The samples have internal radii equal to 7.5 and 9.8 Å, respectively. By working under well-defined relative humidity (RH) values, water dynamics in IMO-OH was revealed by quasi-elastic spectra as a function of the filling of the interior of the tubes. When one water monolayer is present on the inner surface of the tube, water molecules can jump between neighboring Si-OH sites on the circumference by 2.7 Å. A self-diffusion is then measured with a value (D = 1.4 × 10-5 cm2 s-1) around half of that in bulk water. When water molecules start filling also the interior of the tubes, a strong confinement effect is observed, with a confinement diameter (6 Å) of the same order of magnitude as the radius of the nanotube (7.5 Å). When IMO-OH is filled with water, the H-bond network is very rigid, and water molecules are immobile on the timescale of the experiment. For IMO-OH and IMO-CH3, motions of the hydroxyl groups are also evidenced. The associated relaxation time is of the order of 0.5 ps and is due to hindered rotations of these groups. In the case of IMO-CH3, quasi-elastic spectra and elastic scans are dominated by the motions of methyl groups, making the effect of the water content on the evolution of the signals negligible. It was however possible to describe torsions of methyl groups, with a corresponding rotational relaxation time of 2.6 ps. The understanding of the peculiar behavior of water inside inorganic nanotubes has implications in research areas such as nanoreactors. In particular, the locking of motions inside IMO-OH when it is filled with water prevents its use under these conditions as a nanoreactor, while the interior of the IMO-CH3 cavity is certainly a favorable place for confined chemical reactions to take place.

Realization of the kagome spin ice state in a frustrated intermetallic compound.

Spin ices are exotic phases of matter characterized by frustrated spins obeying local "ice rules," in analogy with the electric dipoles in water ice. In two dimensions, one can similarly define ice rules for in-plane Ising-like spins arranged on a kagome lattice. These ice rules require each triangle plaquette to have a single monopole and can lead to different types of orders and excitations. Using experimental and theoretical approaches including magnetometry, thermodynamic measurements, neutron scattering, and Monte Carlo simulations, we establish HoAgGe as a crystalline (i.e., nonartificial) system that realizes the kagome spin ice state. The system features a variety of partially and fully ordered states and a sequence of field-induced phases at low temperatures, all consistent with the kagome ice rule.

Nanoscale Dynamics and Transport in Highly Ordered Low-Dimensional Water.

Highly ordered and highly cooperative water with properties of both solid and liquid states has been observed by means of neutron scattering in hydrophobic one-dimensional channels with van der Waals diameter of 0.78 nm. We have found that in the initial stages of adsorption water molecules occupy niches close to pore walls, followed later by the filling of the central pore area. Intensified by confinement, intermolecular water interactions lead to the formation of well-ordered hydrogen-bonded water chains and to the onset of cooperative vibrations. On the other hand, the same intermolecular interactions lead to two relaxation processes, the faster of which is the spontaneous position exchange between two water molecules placed 3.2-4 Å from each other. Self-diffusion in an axial pore direction is the result of those spontaneous random exchanges and is substantially slower than the self-diffusion in bulk water.

Conformation-controlled hydrogen storage in the CAU-1 metal-organic framework.

We have studied the mechanism of hydrogen storage in the aluminium based metal-organic framework CAU-1 or [Al4(OH)2(OCH3)4(O2C-C6H3NH2-CO2)3] using a complementary multidisciplinary approach of volumetric gas sorption analysis, in situ neutron diffraction and spectroscopy and ab initio calculations. The structure of CAU-1 forms two different types of microporous cages: (i) an octahedral cage with a diameter of about 10 Å and (ii) a tetrahedral cage with a diameter of about 5 Å. Though all metal sites of CAU-1 are fully coordinated, the material exhibits relatively high storage capacities, reaching 4 wt% at a temperature of 70 K. Our results reveal that hydrogen sorption is dominantly driven by cooperative guest-guest interactions and interactions between guest hydrogen molecules and organic linkers. The adsorption of hydrogen on the organic linkers leads to the contraction of the host framework structure and as a result to changes in the electronic potential surface inside the pores. This, in turn, leads to cooperative rearrangement of the molecules inside the pores and to the formation of additionally occupied positions, increasing hydrogen uptake. At the final stage we observe the formation of solid amorphous hydrogen inside the pores.

Intra-cage dynamics of molecular hydrogen confined in cages of two different dimensions of clathrate hydrates.

In porous materials the molecular confinement is often realized by means of weak Van der Waals interactions between the molecule and the pore surface. The understanding of the mechanism of such interactions is important for a number of applications. In order to establish the role of the confinement size we have studied the microscopic dynamics of molecular hydrogen stored in the nanocages of clathrate hydrates of two different dimensions. We have found that by varying the size of the pore the diffusive mobility of confined hydrogen can be modified in both directions, i.e. reduced or enhanced compared to that in the bulk solid at the same temperatures. In the small cages with a mean crystallographic radius of 3.95 Å the confinement reduces diffusive mobility by orders of magnitude. In contrast, in large cages with a mean radius of 4.75 Å hydrogen molecules displays diffusive jump motion between different equilibrium sites inside the cages, visible at temperatures where bulk H2 is solid. The localization of H2 molecules observed in small cages can promote improved functional properties valuable for hydrogen storage applications.

Water dynamics in physical hydrogels based on partially hydrophobized hyaluronic acid.

The dynamics of hyaluronate-based hydrogels has been investigated by quasielastic neutron scattering (QENS). Hyaluronate (HYA) has been compared, in the same conditions of temperature and polymer concentration, to a chemically modified form, HYADD, in which the backbone has been grafted with a hexadecyl (C(16)) side-chain with a degree of substitution of about 2% (mol/mol). This modification increases the hydrophobicity of the polysaccharide and leads to a stable gel already at polymer concentration of 0.3% (w/v), yielding a viscosupplementation with less quantity of polysaccharide. The time-scale covered by our measurements probes both water and segmental biopolymer motions. In both systems, the local dynamics of the network in the ps time-scale is mostly due to local reorientational motions of side groups. Such motions are not significantly affected by the small amount of aliphatic chains forming the hydrophobic junctions in HYADD. The diffusivity of water in both HYA and HYADD coincides with that of pure water within the experimental uncertainty. This result confirms previous ones on the dynamics of water in HYA solutions and it is of relevance for biomedical applications of hyaluronate-based systems because it affects the diffusive processes of metabolites and their interaction with tissues.

Alcohol induced structural and dynamic changes in ß-lactoglobulin in aqueous solution: a neutron scattering study.

Structural and dynamic properties of ß-lactoglobulin (ß-LG) were revealed as a function of alcohol concentration in ethanol- and trifluoroethanol(TFE)-water mixtures with circular dichroism (CD), small-angle neutron scattering (SANS) and quasi-elastic neutron scattering (QENS). The CD spectra showed that an increase in TFE concentration promotes the formation of the ß-sheet structure of ß-LG. The SANS-intensities were fitted using form factors for two attached spheres for the native and native-like states of the protein. At higher alcohol concentrations, where aggregation takes place, a form factor modelling diffusion limited colloidal aggregation (DLCA) was employed. The QENS-data were analyzed in terms of internal motions for all alcohol concentrations. While low concentrations of TFE (10% (v/v)) lead to an increase of the mean square amplitudes of vibrations and a retention of a native-like structure - but not to an increase of the characteristic radius of proton diffusion processes a. Addition of 20% (v/v) of TFE induces aggregation, going along with a further increase of . Further increase of TFE concentration to 30% (v/v) changes the nanoscale structure of the oligomeric nucleate, but induces no further significant changes in . The present study underlines the necessity of methods sensitive to the dynamics of a system to obtain a complete picture of a molecular process.

BH4− self-diffusion in liquid LiBH4.

The hydrogen dynamics in solid and in liquid LiBH4 was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH4- subunits on the picosecond scale was observed in solid LiBH4. The characteristic time constant is significantly shortened when the system transforms from the low-temperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH4 above 553 K, translational diffusion of the BH4- units is found. The measured diffusion coefficients are in the 10(-5)cm2/s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of Ea = 88 meV and a prefactor of D0 = 3.1 × 10(-4)cm2/s.

Temperature dependence of the primary relaxation in 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide.

We present results from complementary characterizations of the primary relaxation rate of a room temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} imide, [C6mim][Tf2N], over a wide temperature range. This extensive data set is successfully merged with existing literature data for conductivity, viscosity, and NMR diffusion coefficients thus providing, for the case of RTILs, a unique description of the primary process relaxation map over more than 12 decades in relaxation rate and between 185 and 430 K. This unique data set allows a detailed characterization of the VTF parameters for the primary process, that are: B=890 K, T0=155.2 K, leading to a fragility index m=71, corresponding to an intermediate fragility. For the first time neutron spin echo data from a fully deuteriated sample of RTIL at the two main interference peaks, Q=0.76 and 1.4 A(-1) are presented. At high temperature (T>250 K), the collective structural relaxation rate follows the viscosity behavior; however at lower temperatures it deviates from the viscosity behavior, indicating the existence of a faster process.