Structure of water-in-salt and water-in-bisalt electrolytes.

We report a systematic diffraction study of two "water-in-salt" electrolytes and a "water-in-bisalt" electrolyte combining high-energy X-ray diffraction (HEXRD) with polarized and unpolarized neutron diffraction (ND) on both H2O and D2O solutions. The measurements provide three independent combinations of correlations between the different pairs of atom types that reveal the short- and intermediate-range order in considerable detail. The ND interference functions show pronounced peaks around a scattering vector Q ∼ 0.5 Å-1 that change dramatically with composition, indicating significant rearrangements of the water network on a length scale around 12 Å. The experimental results are compared with two sets of Molecular Dynamics (MD) simulations, one including polarization effects and the other based on a non-polarizable force field. The two simulations reproduce the general shapes of the experimental structure factors and their changes with concentration, but differ in many detailed respects, suggesting ways in which their force fields might be modified to better represent the actual systems.

Local Structures of Oxygen-Deficient Perovskite Sr2ScGaO5 Polymorphs Explored by Total Neutron Scattering and EXAFS Spectroscopy.

Depending on the synthesis route, the oxygen ion electrolyte Sr2ScGaO5 shows two polymorphs, a brownmillerite and a cubic perovskite framework. In order to better explore oxygen diffusion pathways and mechanisms, we report here on a multitechnical approach to characterize local structural changes for Sr2ScGaO5 polymorphs as a function of temperature, using a neutron pair distribution function (PDF) analysis together with an extended X-ray absorption fine structure (EXAFS) analysis. While for the brownmillerite type structure PDF and Rietveld refinements yield identical structural descriptions, considerable differences are found for the cubic oxygen-deficient polymorph. On a local scale a brownmillerite type vacancy structure could be evidenced for the cubic phase, suggesting a complex short-range ordering and respective microstructure. Both PDF and EXAFS data confirm an octahedral and tetrahedral coordination for Sc and Ga, respectively, at a local scale for both polymorphs. Related changes in the bond distances and oxygen vacancy ordering are discussed.

Impact of Amorphization Methods on the Physicochemical Properties of Amorphous Lactulose.

The influence of the amorphization technique on the physicochemical properties of amorphous lactulose was investigated. Four different amorphization techniques were used: quenching of the melt, milling, spray-drying, and freeze-drying, and amorphous samples were analyzed by differential scanning calorimetry, NMR spectroscopy, and powder X-ray diffraction analysis. Special attention was paid to the tautomeric composition and to the glass transition of amorphized materials. It was found that the tautomeric composition of the starting physical state (crystal, liquid, or solution) is preserved during the amorphization process and has a strong repercussion on the glass transition of the material. The correlation between these two properties as well as the plasticizing effect of the different tautomers was clarified by molecular dynamics simulations.

Morphological and Structural Properties of Amorphous Lactulose Studied by Scanning Electron Microscopy, Polarized Neutron Scattering, and Molecular Dynamics Simulations.

Morphological and structural properties of amorphous disaccharide lactulose (C12H22O11), obtained by four different amorphization methods (milling, quenching of the melt form, spray-drying, and freeze-drying), are investigated by scanning electron microscopy, polarized neutron scattering, and molecular dynamics simulations. While major differences on the morphology of the different amorphous samples are revealed by scanning electron microscopy images, only subtle structural differences have been found by polarized neutron scattering. Microstructure of the milled sample appears slightly different from the other amorphized materials with the presence of remaining crystalline germs which are not detected by X-ray diffraction. Quantitative phase analysis shows that these remaining crystallites are present in a ratio between 1 and 4%, and their size remains between 20 and 30 nm despite a long milling time of about 8 h. The impact of the change in tautomeric concentrations on the physical properties of lactulose in the amorphous state has been investigated from molecular dynamics simulations. It is suggested that chemical differences between lactulose tautomers could be at the origin of small structural differences detected by polarized neutron scattering.

Lactulose: A Model System to Investigate Solid State Amorphization Induced by Milling.

In this article, we show that crystalline lactulose can be amorphized directly in the solid state by mechanical milling. Moreover, compared to similar materials, the amorphization kinetics of lactulose is found to be very rapid and the amorphous state thus obtained appears to be very stable against recrystallization on heating. These features make lactulose a model compound for this type of solid state transformation. The ease of crystalline lactulose to be amorphized on milling is explained by comparing elastic constants of lactulose with those of several other disaccharides. These constants have been determined by molecular dynamics simulations. The article also shows how isothermal dissolution calorimetry can be used effectively for the determination of amorphization kinetics during grinding when the usual characterization techniques (differential scanning calorimetry and powder X-ray diffraction) fail.

Stabilization of Metastable Thermoelectric Crystalline Phases by Tuning the Glass Composition in the Cu-As-Te System.

Recrystallization of amorphous compounds can lead to the stabilization of metastable crystalline phases, which offers an interesting way to unveil novel binary or ternary compounds and control the transport properties of the obtained glass ceramics. Here, we report on a systematic study of the Cu-As-Te glassy system and show that under specific synthesis conditions using the spark-plasma-sintering technique, the α-As2Te3 and ß-As2Te3 binary phases and the previously unreported AsTe3 phase can be selectively crystallized within an amorphous matrix. The microstructures and transport properties of three different glass ceramics, each of them containing one of these phases with roughly the same crystalline fraction (∼30% in volume), were investigated in detail by means of X-ray diffraction, scanning electron microscopy, neutron thermodiffraction, Raman scattering (experimental and lattice-dynamics calculations), and transport-property measurements. The physical properties of the glass ceramics are compared with those of both the parent glasses and the pure crystalline phases that could be successfully synthesized. SEM images coupled with Raman spectroscopy evidence a "coast-to-island" or dendriticlike microstructure with microsized crystallites. The presence of the crystallized phase results in a significant decrease in the electrical resistivity while maintaining the thermal conductivity to low values. This study demonstrates that new compounds with interesting transport properties can be obtained by recrystallization, which in turn provides a tuning parameter for the transport properties of the parent glasses.

Neutrons describe ectoine effects on water H-bonding and hydration around a soluble protein and a cell membrane.

Understanding adaptation to extreme environments remains a challenge of high biotechnological potential for fundamental molecular biology. The cytosol of many microorganisms, isolated from saline environments, reversibly accumulates molar concentrations of the osmolyte ectoine to counterbalance fluctuating external salt concentrations. Although they have been studied extensively by thermodynamic and spectroscopic methods, direct experimental structural data have, so far, been lacking on ectoine-water-protein interactions. In this paper, in vivo deuterium labeling, small angle neutron scattering, neutron membrane diffraction and inelastic scattering are combined with neutron liquids diffraction to characterize the extreme ectoine-containing solvent and its effects on purple membrane of H. salinarum and E. coli maltose binding protein. The data reveal that ectoine is excluded from the hydration layer at the membrane surface and does not affect membrane molecular dynamics, and prove a previous hypothesis that ectoine is excluded from a monolayer of dense hydration water around the soluble protein. Neutron liquids diffraction to atomic resolution shows how ectoine enhances the remarkable properties of H-bonds in water-properties that are essential for the proper organization, stabilization and dynamics of biological structures.

Polymorphism in Thermoelectric As2Te3.

Metastable ß-As2Te3 (R3̅m, a = 4.047 Å and c = 29.492 Å at 300 K) is isostructural to layered Bi2Te3 and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As2Te3 leads to multiphase samples, while ß-As2Te3 could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, ß-As2Te3 reconstructively transforms into stable α-As2Te3 (C2/m, a = 14.337 Å, b = 4.015 Å, c = 9.887 Å, and ß = 95.06°) at 480 K. This ß â†’ α transformation can be seen as the displacement of part of the As atoms from their As2Te3 layers into the van der Waals bonding interspace. Upon cooling, ß-As2Te3 displacively transforms in two steps below T(S1) = 205-210 K and T(S2) = 193-197 K into a new ß'-As2Te3 allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic ß'-As2Te3 crystal structure (P2(1)/m, a = 6.982 Å, b = 16.187 Å, c = 10.232 Å, ß = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by ß-As2Te3 is accompanied by a 4-fold modulation along its b axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the ß'-phase being more stable than the ß-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As2Te3 layers in all three structures.

Comparison of the atomic level structure of the plastic crystalline and liquid phases of CBr2Cl2: neutron diffraction and reverse Monte Carlo modelling.

Neutron diffraction results obtained for plastic crystalline dichlorodibromomethane (CBr2Cl2) have been modelled by means of the reverse Monte Carlo method. Comparison with its liquid phase is provided at several levels of the atomic structure (total scattering structure factors, partial radial distribution functions, orientational and dipole-dipole correlations). The results reveal that the relative orientation of neighbouring molecules largely depends on the steric effect. The small dipole moment does not have as strong an influence as the steric effect on the short-range order. Our observations fit well with earlier findings presented for the series CBr(n)Cl(4-n) (n = 0, 1, 2, 4).

Anion disorder in lanthanoid zirconates Gd(2-x)Tb(x)Zr2O7.

The pyrochlore-defect fluorite order-disorder transition has been studied for a series of oxides of the type Gd(2-x)Tb(x)Zr2O7 by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at λ ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore-defect fluorite boundary of Ln2Zr2O7 is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order-disorder transition across the Gd(2-x)Tb(x)Zr2O7 series.

Unraveling the atomic structure of Ge-rich sulfide glasses.

In contrast to the well-established structure of glassy GeS2, consisting of corner- and edge-sharing GeS(4/2) tetrahedra, the structural features of Ge-rich sulfide alloys remain essentially unknown. Two contrasting points of view: (1) a tetrahedral model, and (2) a distorted NaCl approach were neither confirmed nor excluded mostly because of missing advanced structural studies. Using high-energy X-ray scattering and neutron diffraction, we show the complexity of the short and intermediate range order in Ge(x)S(1-x) glasses, ⅓ ≤ x ≤ 0.47, formed by corner- and edge-sharing tetrahedra with two-fold coordinated sulfur species and a variable number of Ge-Ge bonds, and Ge-S units with three-fold coordinated sulfur at x ≥ 0.36.

Structural characterisation of hypoxia-mimicking bioactive glasses.

Nickel and cobalt are both known to stimulate the hypoxia-inducible factor-1 (HIF-1α), thus significantly improving blood vessel formation in tissue engineering applications. We have manufactured nickel and cobalt doped bioactive glasses to act as a controlled delivery mechanism of these ions. The resultant structural consequences have been investigated using the methods of isotopic and isomorphic substitution applied to neutron diffraction. The structural sites present will be intimately related to their release properties in physiological fluids such as plasma and saliva, and hence the bioactivity of the material. Detailed structural knowledge is therefore a prerequisite for optimising material design. Results show that nickel and cobalt adopt a mixed structural role within these bioactive glasses occupying both network-forming (tetrahedral) and network-modifying (5-fold) geometries. Two thirds of the Ni (or Co) occupies a five-fold geometry with the remaining third in a tetrahedral environment. A direct comparison of the primary structural correlations (e.g. Si-O, Ca-O, Na-O and O-Si-O) between the archetypal 45S5 Bioglass® and the Ni and Co glasses studied here reveal no significant differences. This indicates that the addition of Ni (or Co) will have no adverse effects on the existing structure, and thus on in vitro/in vivo dissolution rates and therefore bioactivity of these glasses.

Structural characterization of titanium-doped Bioglass using isotopic substitution neutron diffraction.

Melt quenched silicate glasses containing calcium, phosphorus and alkali metals have the ability to promote bone regeneration and to fuse to living bone. Of these glasses 45S5 Bioglass® is the most widely used being sold in over 35 countries as a bone graft product for medical and dental applications; particulate 45S5 is also incorporated into toothpastes to help remineralize the surface of teeth. Recently it has been suggested that adding titanium dioxide can increase the bioactivity of these materials. This work investigates the structural consequences of incorporating 4 mol% TiO(2) into Bioglass® using isotopic substitution (of the Ti) applied to neutron diffraction and X-ray Absorption Near Edge Structure (XANES). We present the first isotopic substitution data applied to melt quench derived Bioglass or its derivatives. Results show that titanium is on average surrounded by 5.2(1) nearest neighbor oxygen atoms. This implies an upper limit of 40% four-fold coordinated titanium and shows that the network connectivity is reduced from 2.11 to 1.97 for small quantities of titanium. Titanium XANES micro-fluorescence confirms the titanium environment is homogenous on the micron length scale within these glasses. Solid state magic angle spinning (MAS) NMR confirms the network connectivity model proposed. Furthermore, the results show the intermediate range order containing Na-O, Ca-O, O-P-O and O-Si-O correlations are unaffected by the addition of small quantities of TiO(2) into these systems.

Arsenate incorporation in gypsum probed by neutron, X-ray scattering and density functional theory modeling.

The ability of gypsum, a common sulfate mineral, to host arsenic atoms in its crystalline structure, is demonstrated through experimental structural studies of the solid solutions formed upon synthetic coprecipitation of gypsum (CaSO4 x 2 H2O) and arsenic. Neutron and X-ray diffraction methods show an enlargement of the gypsum unit cell proportional to the concentration of arsenic in the solids and to the pH solution value. The substitution of sulfate ions (SO4(2-)) by arsenate ions is shown to be more likely under alkaline conditions, where the HAsO4(2-) species predominates. A theoretical Density Functional Theory model of the arsenic-doped gypsum structure reproduces the experimental volume expansion. Extended X-ray Absorption Fine Structure (EXAFS) measurements of the local structure around the arsenic atom in the coprecipitated solids confirm solid state substitution and allow some refinement of the local structure, corroborating the theoretical structure found in the simulations. The charge redistribution within the structure upon substitutions of either the protonated or the unprotonated arsenate species studied by means of Mulliken Population Analyses demonstrates an increase in the covalency in the interaction between Ca(2+) and AsO4(3-), whereas the interaction between Ca(2+) and HAsO4(2-) remains predominantly ionic.

Hydration of Hg2+ in aqueous solution studied by neutron diffraction with isotopic substitution.

The structural parameters of Hg2+ hydration were studied in 0.225 mol/L solutions of Hg2+ in DNO3/D2O by means of neutron diffraction with isotopic substitution of 199Hg for natHg. It was found that Hg2+ is hydrated by a first solvation shell of six water molecules. The observed Hg-O and Hg-H distances are equal to 2.48+/-0.05 and 3.08+/-0.05 A, respectively. The angle phi between the plane of the water molecule and the cation-water oxygen axis is approximately 35 degrees . The solvation of Hg2+ therefore mimics very closely that of Ca2+ (the Ca-O and Ca-H distances are 2.40 and 3.03 A, respectively) and helps to account for the extreme toxicity of mercury(II). We note also that the Hg-O distance obtained in the neutron diffraction experiment is larger by approximately 0.1 A than that obtained by X-ray diffraction. This difference is consistent with a shift of the oxygen electron density toward the mercury cation due to the covalency of the Hg-O interaction.

Neutron powder diffraction with (nat)Sm: crystal structures and magnetism of a binary samarium deuteride and a ternary samarium magnesium deuteride.

Binary SmH(3) (trigonal, a=656.7(3), c=680.1(3) pm, P$\bar 3$c1, Z=6), ternary SmMg2H7 (tetragonal, a=626.47(6), c=937.2(2) pm, P4(1)2(1)2, Z=4) and the corresponding deuterides SmD3 (a=653.9(1)m, c=676.7(2) pm) and SmMg2D7 (a=624.10(1), c=934.81(2) pm) have been prepared by hydrogenation (deuteration) of elemental samarium and the Laves phase SmMg2, respectively, and investigated by X-ray and neutron powder diffraction and SQUID and vibration magnetometry. The problem of the enormous neutron absorption of the natural isotopic mixture (natSm) is circumvented by carefully choosing the neutron wavelength (approximately 50 pm) and the use of double-walled cylindrical sample holders and a high-intensity neutron diffractometer (D4c at ILL). SmD3 crystallises with a tysonite-type structure and has three independently ordered deuterium atom sites with trigonal-planar, trigonal-pyramidal and tetrahedral metal environments and Sm--D bond lengths in the range 220(1)-258(1) pm (average: 235 pm). SmMg2D7 is a new deuteride that crystallises with an LaMg2D7-type structure. It displays four fully occupied deuterium sites having triangular and tetrahedral metal environments and Sm--D bond lengths in the range 227.6(5)-246.8(8) pm (average: 239 pm). These are the first samarium-deuterium bond lengths to be reported. Both deuterides are paramagnetic down to 2 K (SmD3: mueff=0.63(1) muB, thetap approximately -4 K; SmMg2D7: mueff=0.57(2) muB, thetap approximately -4 K). Their crystal structures and chemical and physical properties suggest mainly ionic bonding according to the limiting ionic formulae Sm3+(H-)3 and Sm3+(Mg2+)2(H-)7.

Structure of glassy GeO2.

The full set of partial structure factors for glassy germania, or GeO2, were accurately measured by using the method of isotopic substitution in neutron diffraction in order to elucidate the nature of the pair correlations for this archetypal strong glass former. The results show that the basic tetrahedral Ge(O1/2)4 building blocks share corners with a mean inter-tetrahedral Ge-Ô-Ge bond angle of 132(2)°. The topological and chemical ordering in the resultant network displays two characteristic length scales at distances greater than the nearest neighbour. One of these describes the intermediate range order, and manifests itself by the appearance of a first sharp diffraction peak in the measured diffraction patterns at a scattering vector kFSDP≈1.53 Å-1, while the other describes so-called extended range order, and is associated with the principal peak at kPP = 2.66(1) Å-1. We find that there is an interplay between the relative importance of the ordering on these length scales for tetrahedral network forming glasses that is dominated by the extended range ordering with increasing glass fragility. The measured partial structure factors for glassy GeO2 are used to reproduce the total structure factor measured by using high energy x-ray diffraction and the experimental results are also compared to those obtained by using classical and first principles molecular dynamics simulations.

Neutron diffraction and simulation studies of the exocyclic hydroxymethyl conformation of glucose.

The techniques of neutron diffraction with isotopic substitution (NDIS) and molecular dynamics (MD) simulations have been used to examine the rotational conformation of the exocyclic hydroxymethyl group of D-glucopyranose. First order H/D NDIS experiments were performed on the H6 position in 3m aqueous glucose solutions where the average coherent scattering length of the exchangeable hydrogen atoms was zero (i.e., all correlations between exchangeable hydrogen atoms and other atoms cancel and thus are not present in the scattering data). This H6 experimental result suggests that no single conformation for the C4-C5-C6-O6 dihedral reproduces the observed scattering data well, but that a mixture of the gg and gt conformations, which has been suggested by NMR experiments, gives a reasonable agreement between the MD and experimental data.

Glass fragility and atomic ordering on the intermediate and extended range.

The relation between the fragility of glass-forming systems, a parameter which describes many of their key physical characteristics, and atomic scale structure is investigated by using neutron diffraction to measure the topological and chemical ordering for germania, or GeO(2), which is an archetypal strong glass former. We find that the ordering for this and other tetrahedral network-forming glasses at distances greater than the nearest neighbor can be rationalized in terms of an interplay between the relative importance of two length scales. One of these is associated with an intermediate range, the other with an extended range and, with increasing glass fragility, it is the extended range ordering which dominates.

Structure of dense hydrogen fluoride gas from neutron diffraction and molecular dynamics simulations.

The gas phase of hydrogen fluoride has been investigated by neutron diffraction experiments at three different particle densities. All investigated states are within the liquid-gas coexistence region of hydrogen fluoride. From the obtained diffraction data we deduced information about the local structure of the gas phase, which consists of small agglomerates. This has been expected as liquid hydrogen fluoride forms the strongest hydrogen bonds known. Molecular dynamics simulations with a modified potential have been carried out for all experimentally investigated states. The results confirmed that the size of the formed agglomerates in the gas phase is growing with increasing density of the gas phase.