Isotopic Substitution In Neutron Diffraction Research Articles

Structural analysis of potassium borate solutions.

In this work, H/D isotopic substitution neutron diffraction was combined with empirical potential structure refinement (EPSR) and DFT-based quantum calculations to study the interactions between B(OH)3 boric acid molecules, B(OH)4- metaborate ions, water molecules, and potassium cations in borate solutions. The results show that the solute ions and molecules have a marked effect on the second coordination shell of the water molecules, causing a greater deviation from a tetrahedral structure than is observed for pure water. Potassium ions and trans-B(OH)3 tend to form a monodentate contact ion pair (MCIP) with a K-B distance ∼3.8 Å, which remains constant upon changing the solution concentration. Potassium ions and cis-B(OH)3 form both a MCIP at K-B ∼3.8 Å and a bidentate contact ion pair (BCIP) at K-B ∼3.4 Å. As the solution concentration increases, there is a BCIP to MCIP transformation. Boric acid molecules can undergo hydration in one of three ways: direct hydration, interstitial hydration, and axial hydration. The energetic hydration preference is direct hydration → interstitial hydration → axial hydration. Nine water molecules are required when all water molecules directly interact with the -OH groups of B(OH)4-, and a tenth water molecule is located at an interstitial position. The hydrogen bonding between boric acid molecule/metaborate ion and water molecules is stronger than that between water molecules in the hydration layer.

Elucidating the guest disorder in structure II argon hydrate – A neutron diffraction isotopic substitution study

Clathrate hydrates with the cubic structure II (CS-II) form typically with large guest molecules, such as tetrahydrofuran, trimethylamine oxide, or propane. However, CS-II is also realized for argon hydrate despite the comparatively small van der Waals diameter of the guest (around 3.8 ​Å). Here, the structure of deuterated argon hydrate was studied at ambient pressure in the temperature range 20–95 ​K using neutron diffraction and comparing natural Ar with 36Ar, which scatters neutrons more than 13 times more efficiently. The procedure allowed to unambiguously establish the positional disorder within the large cages of CS-II, while simultaneously refining host and guest structures. These cages are singly occupied and off-centered argon atoms distribute on two tetrahedron-shaped split positions with a ratio 3:1. Molecular dynamics (MD) simulations revealed that the crystallographic positional disorder structure is due to mobile argon atoms even at 20 ​K. The MD potential energy distribution confirmed the diffraction model. It is noted that the unit cell volumes of argon hydrate in the investigated temperature range are virtually identical to N2 hydrate, which has a similar composition at ambient pressure, indicating a very similar (slightly attractive) host-guest interaction.

Structural differences between unannealed and expanded high-density amorphous ice based on isotope substitution neutron diffraction

We here report isotope substitution neutron diffraction experiments on two variants of high-density amorphous ice (HDA): its unannealed form prepared via pressure-induced amorphization of hexagonal ice at 77 K, and its expanded form prepared via decompression of very-high density amorphous ice at 140 K. The latter is about 17 K more stable thermally, so that it can be heated beyond its glass-to-liquid transition to the ultraviscous liquid form at ambient pressure. The structural origin for this large thermal difference and the possibility to reach the deeply supercooled liquid state has not yet been understood. Here we reveal that the origin for this difference is found in the intermediate range structure, beyond about 3.6 Å. The hydration shell markedly differs at about 6 Å. The local order, by contrast, including the first as well as the interstitial space between first and second shell is very similar for both. ‘eHDA’ that is decompressed to 0.20 GPa instead of 0.07 GPa is here revealed to be rather far away from well-relaxed eHDA. Instead it turns out to be roughly halfway between VHDA and eHDA – stressing the importance for decompressing VHDA to at least 0.10 GPa to make an eHDA sample of good quality.

The environment of Fe3+/Fe2+ cations in a sodium borosilicate glass

The neutron diffraction isotopic substitution technique is employed to investigate the environment of Fe3+/Fe2+ cations in a sodium borosilicate glass matrix of composition 0.210Na2O0.18511B2O30.605SiO2. The neutron diffraction data were obtained using the D4c diffractometer at the Institut Laue-Langevin (ILL; Grenoble, France), and were recorded for three samples; the base glass, the base glass incorporating natural Fe2O3 (12 mol.%) and a similar glass containing Fe2O3 enriched in 57Fe. The data are Fourier transformed to yield the real-space total correlation function, T(r), and the first co-ordination shells of the Fe3+/Fe2+ cations are investigated via a peak fit to the isotopic difference correlation function TFe(r). It is concluded that the iron is mainly present as Fe3+ cations, both tetrahedrally and octahedrally co-ordinated by oxygen atoms, plus a small fraction (0.07 ± 0.01) of Fe2+ cations in octahedral co-ordination. The Fe3+ tetrahedral fraction is 0.45 ± 0.10, and appears to exist as FeO4 structural units incorporated into the network of silicate chemical groupings, with their negative charge being balanced by Na+ network-modifying cations. The remaining Fe3+ cations (fraction 0.48 ± 0.10) are thought to be predominantly octahedrally co-ordinated and associatedwith BO33 orthoborate anions in FeBO3 chemical groupings, which become non-stoichiometric due to the reduction of some of the Fe3+ cations to Fe2+.

Reverse Monte Carlo investigations concerning recent isotopic substitution neutron diffraction data on liquid water

Although liquid water has been studied for many decades by (X-ray and neutron) diffraction measurements, new experimental results keep appearing, virtually every year. The reason for this is that neither X-ray nor neutron diffraction data are trivial to correct and interpret for this essential substance. Since X-rays are somewhat insensitive to hydrogen, neutron diffraction with (most frequently, H/D) isotopic substitution is vital for investigating the most important feature in water: hydrogen bonding. Here, the two very recent sets of neutron diffraction data are considered, both exploiting the contrast between light and heavy hydrogen, 1H and 2H, in different ways. Reverse Monte Carlo structural modeling is applied for constructing large structural models that are as consistent as possible with all experimental information, both in real and reciprocal spaces. The method has also proven to be useful for revealing where possible small inconsistencies appear during primary data processing: for one neutron data set, it is the molecular geometry that may not be maintained within reasonable limits, whereas for the other set, it is one of the (composite) radial distribution functions that cannot be modeled at the same (high) level as the other three functions. Nevertheless, details of the local structure around the hydrogen bonds appear very much the same for both data sets: the most probable hydrogen bond angle is straight, and the nearest oxygen neighbors of a central oxygen atom occupy approximately tetrahedral positions.

Structure of Molten CaSiO3: Neutron Diffraction Isotope Substitution with Aerodynamic Levitation and Molecular Dynamics Study

We have performed neutron diffraction isotopic substitution experiments on aerodynamically levitated droplets of CaSiO(3), to directly extract intermediate and local structural information on the Ca environment. The results show a substantial broadening of the first Ca-O peak in the pair distribution function of the melt compared to the glass, which comprises primarily of 6- and 7-fold coordinated Ca-polyhedra. The broadening can be explained by a redistribution of Ca-O bond lengths, especially toward longer distances in the liquid. The first order neutron difference function provides a test of recent molecular dynamics simulations and supports the MD model which contains short chains or channels of edge shared Ca-octahedra in the liquid state. It is suggested that the polymerization of Ca-polyhedra is responsible for the fragile viscosity behavior of the melt and the glass forming ability in CaSiO(3).

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.

Intermediate range order in vitreous silica from a partial structure factor analysis

By combining the methods of isotopic substitution in neutron diffraction and high-energy x-ray diffraction, we have determined partial structure factors of vitreous ${\text{SiO}}_{2}$. A discussion of the effect of systematic and statistical errors is presented. The experimental results are found to be in good (but not exact) agreement with existing ab initio and classical molecular-dynamics simulations. No first sharp diffraction peak (FSDP) is observed in the concentration-concentration partial structure factor, ruling out the void-cluster-based model as a possible explanation for the origin of intermediate range order. However, the data are consistent with a model in which the intermediate range order arises from the periodicity of boundaries between a succession of small cages in the network, and the second diffraction peak is associated with chemical ordering of ${\text{SiO}}_{4}$ tetrahedra within continuous regions of the network between cages. The cage model is used to explain compositional trends in the FSDP height for neutron and x-ray data on ${\text{BeO-SiO}}_{2}$ glasses.

Dynamics and structure of nickel chloride–methanol solutions: quasi-elastic neutronscattering and molecular dynamics simulations

The high-resolution quasi-elastic neutron scattering (QENS) technique has been applied tostudy the translational diffusion of methanol protons in pure methanol (MeOH) at 223and 297 K, and in 0.3 and 1.3 molal non-aqueous electrolyte solutions (NAESs) ofNiCl2 in methanol at 297 K. Molecular dynamics (MD) simulations, in conjunction with thepresent QENS results and our previously published structural results obtained by neutrondiffraction isotopic substitution (NDIS) experiments, have been carried out in theNVT ensemble to explore the particle dynamics and microscopic structures ofthe experimentally investigated systems. The simulated structure of the∼1.35 molal NiCl2–MeOH NAES has been compared with the structures ofNi2+ andCl− coordinationshells in ∼1.4 molal NAES obtained earlier by the NDIS technique.

Structure of glassy GeO2

The full set of partial structure factors for glassy germania, orGeO2, were accurately measured by using the method of isotopic substitution inneutron diffraction in order to elucidate the nature of the pair correlations forthis archetypal strong glass former. The results show that the basic tetrahedralGe(O1/2)4 building blocks share corners with a mean inter-tetrahedral Ge–Ô–Ge bond angle of132(2)°. The topological and chemical ordering in the resultant network displays two characteristiclength scales at distances greater than the nearest neighbour. One of these describesthe intermediate range order, and manifests itself by the appearance of a firstsharp diffraction peak in the measured diffraction patterns at a scattering vectorkFSDP≈1.53 Å−1,while the other describes so-called extended range order, and is associated with the principal peakat kPP = 2.66(1) Å−1. We find that there is an interplay between the relative importance of the ordering on theselength scales for tetrahedral network forming glasses that is dominated by the extended rangeordering with increasing glass fragility. The measured partial structure factors for glassyGeO2 are used to reproduce the total structure factor measured by using high energy x-raydiffraction and the experimental results are also compared to those obtained by usingclassical and first principles molecular dynamics simulations.

In vitro changes in the structure of a bioactive calcia–silica sol–gel glass explored using isotopic substitution in neutron diffraction

Bioactive sol–gel derived glass scaffolds bond to bone and their dissolution products stimulate new bone growth in vitro and in vivo; they may therefore be used to regenerate diseased or damaged bone to its original state and function in bone tissue engineering applications. We seek herein to cast light upon these reaction mechanisms by attempting to quantify changes in the atomic-scale structure of the glass scaffold as a result of in vitro reaction with simulated body fluid (SBF). We report the results of a study using neutron diffraction with isotopic substitution (NDIS) to gain new insights into the nature of the atomic scale calcium environment in bioactive sol–gel glasses. This is augmented by high-energy X-ray total diffraction. We have thereby begun to explore the nature of the principal stages to the generation of hydroxyapatite (i.e. the mineral ‘building block’ of bone) on the bioactive glass surface. The data are examined in light of our complementary solid-state NMR and computer modelling studies. The results reveal that the Ca–O environment in an SBF exposed (CaO) 0.3(SiO 2) 0.7 sol–gel glass, which initially comprises three distinct but partially overlapping correlation shells centered at 2.3 Å, 2.5 Å and 2.75 Å, preferentially loses the shortest length correlation. A Ca⋯H correlation appears at 2.95 Å. The surface deposited Ca⋯P environment consists of three partially overlapping, but nonetheless distinct, correlation shells, at 3.15 Å, 3.40 Å and 3.70 Å.

Determination of Fe3+ sites in a NaFeSi2O6 glass by neutron diffraction with isotopic substitution coupled with numerical simulation

Isotopic substitution in neutron diffraction combined with empirical potential structure refinement simulations has been employed to probe the sites occupied by Fe3+ and Fe2+ in a NaFeSi2O6 glass. This study reveals the presence of two populations of Fe3+ sites. 95% of Fe3+ is in tetrahedral sites (dFe3+–O=1.866±0.001Å), as the remainder ferric iron and ferrous iron (∼20% of total Fe) are five coordinated.

Structural Characteristics of a 0.23 Mole Fraction Aqueous Solution of Tetrahydrofuran at 20 °C

Hydrogen/deuterium isotopic substitution neutron diffraction techniques were used to measure the structural correlation functions in a 0.23 mole fraction solution of tetrahydrofuran in water at room temperature. Empirical potential structure refinement (EPSR) was used to build a three-dimensional model of the liquid structure that is consistent with the experimental data. Detailed analysis shows a preference for nonpolar interactions between the cyclic ether molecules plus polar interactions between the ether and solvent water and hydrophobic hydration of the nonpolar regions of the solute. The increase in the number of hydrogen-bond-acceptor sites relative to the number of hydrogen-bond-donor sites in this system, compared to the balanced situation that would be found in pure water, has a marked compressive effect on the structure of the solvent. Despite the small size of the solvent water molecules, the 0.23 mole fraction aqueous solution is still found to contain small voids akin to those in pure liquid tetrahydrofuran. In contrast to the positive surface charge of the voids in the pure system, the average void in this aqueous solution is found to have a net negative charge. This is due to contributions from the water oxygen atoms that are negatively polarized by their intramolecular bonding.

Topological changes in glassyGeSe2at pressures up to9.3GPadetermined by high-energy x-ray and neutron diffraction measurements

Monochromatic high-energy x-ray diffraction measurements employing microfocusing optics were performed on glassy $\mathrm{Ge}{\mathrm{Se}}_{2}$ in a diamond anvil cell at pressures up to $9.3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. In addition, the method of isotopic substitution in neutron diffraction was applied to samples that had been densified by 4% via pressurization to $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in a multianvil device and subsequently recovered to ambient conditions. The results reveal a steady increase with pressure of the average coordination number of Ge from 4.0(2) under ambient conditions to 4.5(2) at $9.3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. With increasing pressure, the first sharp diffraction peak in the measured diffraction patterns at $\ensuremath{\sim}1.0\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$ decreases in intensity and almost disappears while the amplitude of the peaks beyond the nearest neighbor in the measured total pair distribution functions gradually increases. Equation of state measurements show a gradual density increase of 33% from ambient pressure to $8.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ which is in good agreement with molecular dynamics simulations. The results are consistent with the occurrence of two densification processes for glassy $\mathrm{Ge}{\mathrm{Se}}_{2}$, namely, a conversion from edge-sharing to corner-sharing tetrahedra and a gradual increase in the average local coordination number with increasing density.

The Structure of Liquid Tetrahydrofuran

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the structural correlation functions of liquid tetrahydrofuran at room temperature. Empirical potential structure refinement (EPSR) has been used to build a three-dimensional model of the liquid structure that is consistent with the experimental data. Analysis to the level of the orientational correlation functions shows that the liquid displays a preference for T-like configurations between the tetrahydrofuran molecules, a local structure that results in void-like regions of approximately 1.25 angstroms radius within the bulk liquid. The surface chemistry of these voids suggests a slightly positive electrostatic character. These findings are consistent with the known propensity of the liquid to solvate free electrons.

Isotopic substitution neutron diffraction for enhanced structural information from crystalline powder materials

Isotopic substitution neutron diffraction for enhanced structural information from crystalline powder materials

Topological versus chemical ordering in network glasses at intermediate and extended length scales

Atomic ordering in network glasses on length scales longer than nearest-neighbour length scales has long been a source of controversy. Detailed experimental information is therefore necessary to understand both the network properties and the fundamentals of glass formation. Here we address the problem by investigating topological and chemical ordering in structurally disordered AX2 systems by applying the method of isotopic substitution in neutron diffraction to glassy ZnCl2. This system may be regarded as a prototypical ionic network forming glass, provided that ion polarization effects are taken into account, and has thus been the focus of much attention. By experiment, we show that both the topological and chemical ordering are described by two length scales at distances greater than nearest-neighbour length scales. One of these is associated with the intermediate range, as manifested by the appearance in the measured diffraction patterns of a first sharp diffraction peak at 1.09(3) A(-1); the other is associated with an extended range, which shows ordering in the glass out to 62(4) A. We also find that these general features are characteristic of glassy GeSe2, a prototypical covalently bonded network material. The results therefore offer structural insight into those length scales that determine many important aspects of supercooled liquid and glass phenomenology.

Structure of Nd-doped glasses measured by isotopic substitution in neutron diffraction

The structural environment of Nd ions in glasses can have a significant effect on their performance in optical devices. Isotopic substitution in neutron diffraction has been used to probe the structure of Nd2O3 doped SiO2 and La2O3+Y2O3+Al2O3 glasses. In the silicate glass, Nd is found to be ∼sixfold coordinated by oxygen atoms at a distance of 2.28(2) Å with additional Nd–O/Si correlations at 3.55(3), 4.78(3), 5.89(3) Å, and higher distances. In the rare-earth aluminate glass a peak at 2.49(2) Å and a shoulder at ∼3 Å indicate the presence of 8 or higher coordinate Nd polyhedra by comparison with bond valence theory.

Determination of the cation distribution in Fe2Ni(PO4)2using isotopic substitution and powder neutron diffraction

The distribution of divalent iron and nickel over the two metal sites with differing coordination geometry in Fe2Ni(PO4)2, sarcopside, has been investigated using time-of-flight powder neutron diffraction of nickel isotopically substituted materials. Data from four separate samples were collected using HRPD at ISIS, containingnatNi,58Ni,60Ni and62Ni, under identical conditions. The occupancy of iron on theM(1) site was found to be 0.290 (1) from a combined-data-set Rietveld refinement of the three isotopically substituted samples, compared with 0.26 (4) and 0.26 (15) respectively from this and a previous time-of-flight powder neutron diffraction study using natural-abundance nickel, and 0.366 (6) and 0.376 (3) using anomalous X-ray scattering techniques. A critical comparison of isotope substitution neutron diffraction and anomalous X-ray scattering methods for distinguishing nickel and iron from powder data is presented.

Multiple-data-set Rietveld analysis using isotopes in powder neutron diffraction. 2. Positional and anisotropic thermal displacement parameter determinations in CuO (tenorite) at 2 K and 300 K

The technique of isotope-substitution neutron diffraction (ISND) and combined-data-set Rietveld analysis from powder neutron data of crystalline materials is presented and compared with single-data-set powder refinement methods. The rationale behind improvements in the precision and accuracy of the refined model as a result of reduction in parameter correlation in the least-squares technique is described. The improvements are demonstrated practically through a study of isotopically copper-substituted tenorite, CuO, at 2 and 300 K. Typically, the estimated errors on structural parameters from the combined analysis technique are 30% lower than separate single-data-set analyses. Comparison of the precision and accuracy of the structural models obtained from this investigation with previous single-crystal X-ray studies are also presented.