Plutonium oxide melt structure and covalency.

Advances in nuclear power reactors include the use of mixed oxide fuel, containing uranium and plutonium oxides. The high-temperature behaviour and structure of PuO2-x above 1,800 K remain largely unexplored, and these conditions must be considered for reactor design and planning for the mitigation of severe accidents. Here, we measure the atomic structure of PuO2-x through the melting transition up to 3,000 ± 50 K using X-ray scattering of aerodynamically levitated and laser-beam-heated samples, with O/Pu ranging from 1.57 to 1.76. Liquid structural models consistent with the X-ray data are developed using machine-learned interatomic potentials and density functional theory. Molten PuO1.76 contains some degree of covalent Pu-O bonding, signalled by the degeneracy of Pu 5f and O 2p orbitals. The liquid is isomorphous with molten CeO1.75, demonstrating the latter as a non-radioactive, non-toxic, structural surrogate when differences in the oxidation potentials of Pu and Ce are accounted for. These characterizations provide essential constraints for modelling pertinent to reactor safety design.

Trimethylamine-N-oxide depletes urea in a peptide solvation shell.

Trimethylamine-N-oxide (TMAO) and urea are metabolites that are used by some marine animals to maintain their cell volume in a saline environment. Urea is a well-known denaturant, and TMAO is a protective osmolyte that counteracts urea-induced protein denaturation. TMAO also has a general protein-protective effect, for example, it counters pressure-induced protein denaturation in deep-sea fish. These opposing effects on protein stability have been linked to the spatial relationship of TMAO, urea, and protein molecules. It is generally accepted that urea-induced denaturation proceeds through the accumulation of urea at the protein surface and their subsequent interaction. In contrast, it has been suggested that TMAO's protein-stabilizing effects stem from its exclusion from the protein surface, and its ability to deplete urea from protein surfaces; however, these spatial relationships are uncertain. We used neutron diffraction, coupled with structural refinement modeling, to study the spatial associations of TMAO and urea with the tripeptide derivative glycine-proline-glycinamide in aqueous urea, aqueous TMAO, and aqueous urea-TMAO (in the mole ratio 1:2 TMAO:urea). We found that TMAO depleted urea from the peptide's surface and that while TMAO was not excluded from the tripeptide's surface, strong atomic interactions between the peptide and TMAO were limited to hydrogen bond donating peptide groups. We found that the repartition of urea, by TMAO, was associated with preferential TMAO-urea bonding and enhanced urea-water hydrogen bonding, thereby anchoring urea in the bulk solution and depleting urea from the peptide surface.

Microgravity effects on nonequilibrium melt processing of neodymium titanate: thermophysical properties, atomic structure, glass formation and crystallization.

The relationships between materials processing and structure can vary between terrestrial and reduced gravity environments. As one case study, we compare the nonequilibrium melt processing of a rare-earth titanate, nominally 83TiO2-17Nd2O3, and the structure of its glassy and crystalline products. Density and thermal expansion for the liquid, supercooled liquid, and glass are measured over 300-1850 °C using the Electrostatic Levitation Furnace (ELF) in microgravity, and two replicate density measurements were reproducible to within 0.4%. Cooling rates in ELF are 40-110 °C s-1 lower than those in a terrestrial aerodynamic levitator due to the absence of forced convection. X-ray/neutron total scattering and Raman spectroscopy indicate that glasses processed on Earth and in microgravity exhibit similar atomic structures, with only subtle differences that are consistent with compositional variations of ~2 mol. % Nd2O3. The glass atomic network contains a mixture of corner- and edge-sharing Ti-O polyhedra, and the fraction of edge-sharing arrangements decreases with increasing Nd2O3 content. X-ray tomography and electron microscopy of crystalline products reveal substantial differences in microstructure, grain size, and crystalline phases, which arise from differences in the melt processes.

The role of urea in formation of the sodium acetate trihydrate (SAT)-urea eutectic liquid: a neutron diffraction and isotopic substitution study.

Neutron diffraction with isotopic substitution has been used to investigate the structure of the liquid sodium acetate trihydrate-urea eutectic (mole fraction (χurea) of 0.60) at 50 °C. Urea competes with acetate anions and water molecules in the solvation of sodium ions, displacing water and, simultaneously, stabilising the liberated 'excess' water through hydrogen bonding between water and urea molecules in the eutectic liquid. This provides a direct insight into the role of urea as both denaturant and hydrogen-bond network former in generating eutectic liquids.

The structure of molten calcium ferrite under various redox conditions.

Laser-heated melts based on the 43CaO-57Fe2O3-x eutectic, close to the calcium ferrite (CF) composition, were measured with high-energy X-ray diffraction using aerodynamic levitation over a range of redox states controlled by CO/CO2 gas atmospheres. The iron-oxygen coordination number was found to rise from 4.4 ± 0.3 at 15% Fe3+ to 5.3 ± 0.3 at 87% Fe3+. Empirical potential structure refinement modelling was used to obtain the ferric and ferrous partial pair distribution functions. It was found that the Fe2+ iron-oxygen coordination number is consistently approximately 10% higher in CF than in pure iron oxide, while Fe3+ is essentially identical in all but the most oxygen-rich environments (where it is higher in CF compared with FeOx). The model also shows calcium octahedra to be the dominant species across all redox environments, although the population of CaO7 increases with the availability of oxygen at the expense of CaO4 and CaO5. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Octahedral oxide glass network in ambient pressure neodymium titanate.

Rare-earth titanates form very fragile liquids that can be made into glasses with useful optical properties. We investigate the atomic structure of 83TiO2-17Nd2O3 glass using pair distribution function (PDF) analysis of X-ray and neutron diffraction with double isotope substitutions for both Ti and Nd. Six total structure factors are analyzed (5 neutron + 1 X-ray) to obtain complementary sensitivities to O and Ti/Nd scattering, and an empirical potential structure refinement (EPSR) provides a structural model consistent with the experimental measurements. Glass density is estimated as 4.72(13) g cm-3, consistent with direct measurements. The EPSR model indicates nearest neighbor interactions for Ti-O at [Formula: see text] = 1.984(11) Å with coordination of [Formula: see text] = 5.72(6) and for Nd-O at [Formula: see text] = 2.598(22) Å with coordination of [Formula: see text] = 7.70(26), in reasonable agreement with neutron first order difference functions for Ti and Nd. The titanate glass network comprises a mixture of distorted Ti-O5 and Ti-O6 polyhedra connected via 71% corner-sharing and 23% edge-sharing. The O-Ti coordination environments include 15% nonbridging O-Ti1, 51% bridging O-Ti2, and 32% tricluster O-Ti3. This structure is highly unusual for oxide glasses melt-quenched at ambient pressure, as it consists of Ti-Ox predominantly in octahedral (with nearly no tetrahedral) coordination.

The Structure of Liquid and Glassy Carbamazepine.

To enhance the solubility of orally administered pharmaceuticals, liquid capsules or amorphous tablets are often preferred over crystalline drug products. However, little is known regarding the variation in bonding mechanisms between pharmaceutical molecules in their different disordered forms. In this study, liquid and melt-quenched glassy carbamazepine have been studied using high energy X-ray diffraction and modeled using Empirical Potential Structure Refinement. The results show significant structural differences between the liquid and glassy states. The liquid shows a wide range of structures; from isolated molecules, to aromatic ring correlations and NH-O hydrogen bonding. Upon quenching from the liquid to the glass the number of hydrogen bonds per molecule increases by ~50% at the expense of a ~30% decrease in the close contact (non-bonded) carbon-carbon interactions between aromatic rings. During the cooling process, there is an increase in both singly and doubly hydrogen-bonded adjacent molecules. Although hydrogen-bonded dimers found in the crystalline states persist in the glassy state, the absence of a crystalline lattice also allows small, hydrogen-bonded NH-O trimers and tetramers to form. This proposed model for the structure of glassy carbamazepine is consistent with the results from vibrational spectroscopy and nuclear magnetic resonance.

Structural Markers of the Frenkel Line in the Proximity of Widom Lines.

We have performed a neutron scattering experiment on supercritical fluid nitrogen at 160 K (1.27 TC) over a wide pressure range (7.8 MPa/0.260 g/mL-125 MPa/0.805 g/mL). This has enabled us to study the process by which nitrogen changes from a fluid that exhibits gaslike behavior to one that exhibits rigid liquidlike behavior at a temperature close to, but above, the critical temperature by crossing the Widom lines followed by the Frenkel line on pressure (density) increase. We find that the Frenkel line transition is indicated by a transition to a regime of rigid liquidlike behavior in which the coordination number remains constant within error, in agreement with our previous work at 300 K. The Frenkel line transition takes place at approximately the same density at 160 and 300 K. The data do not conclusively show an additional transition at the location of the known Widom lines. We find that behavior remains gaslike until the Frenkel line is crossed and our data support the hypothesis that Widom line transitions are density increase-driven.

Low-Dimensional Network Formation in Molten Sodium Carbonate.

Molten carbonates are highly inviscid liquids characterized by low melting points and high solubility of rare earth elements and volatile molecules. An understanding of the structure and related properties of these intriguing liquids has been limited to date. We report the results of a study of molten sodium carbonate (Na2CO3) which combines high energy X-ray diffraction, containerless techniques and computer simulation to provide insight into the liquid structure. Total structure factors (F(x)(Q)) are collected on the laser-heated carbonate spheres suspended in flowing gases of varying composition in an aerodynamic levitation furnace. The respective partial structure factor contributions to F(x)(Q) are obtained by performing molecular dynamics simulations treating the carbonate anions as flexible entities. The carbonate liquid structure is found to be heavily temperature-dependent. At low temperatures a low-dimensional carbonate chain network forms, at T = 1100 K for example ~55% of the C atoms form part of a chain. The mean chain lengths decrease as temperature is increased and as the chains become shorter the rotation of the carbonate anions becomes more rapid enhancing the diffusion of Na(+) ions.

Lone-pair distribution and plumbite network formation in high lead silicate glass, 80PbO·20SiO2.

For the first time a detailed structural model has been determined which shows how the lone-pairs of electrons are arranged relative to each other in a glass network containing lone-pair cations. High energy X-ray and neutron diffraction patterns of a very high lead content silicate glass (80PbO·20SiO2) have been used to build three-dimensional models using empirical potential structure refinement. Coordination number and bond angle distributions reveal structural similarity to crystalline Pb11Si3O17 and α- and ß-PbO, and therefore strong evidence for a plumbite glass network built from pyramidal [PbO(m)] polyhedra (m ~ 3-4), with stereochemically active lone-pairs, although with greater disorder in the first coordination shell of lead compared to the first coordination shell of silicon. The oxygen atoms are coordinated predominantly to four cations. Explicit introduction of lone-pair entities into some models leads to modification of the local Pb environment, whilst still allowing for reproduction of the measured diffraction patterns, thus demonstrating the non-uniqueness of the solutions. Nonetheless, the models share many features with crystalline Pb11Si3O17, including the O-Pb-O bond angle distribution, which is more highly structured than reported for lower Pb content glasses using reverse Monte Carlo techniques. The lone-pair separation of 2.85 Å in the model glasses compares favourably with that estimated in α-PbO as 2.88 Å, and these lone-pairs organise to create voids in the glass, just as they create channels in Pb11Si3O17 and interlayer spaces in the PbO polymorphs.

Spectral assignments and NMR parameter-structure relationships in borates using high-resolution 11B NMR and density functional theory.

High-resolution, solid-state (11)B NMR spectra have been obtained at high magnetic fields for a range of polycrystalline borates using double-rotation (DOR), multiple-quantum magic angle spinning and isotopic dilution. DOR linewidths can be less than 0.2 ppm in isotopically diluted samples, allowing highly accurate values for the isotropic chemical shift, δiso, and electric field gradient to be obtained. The experimental values are used as a test of density functional calculations using both projector augmented wave based CASTEP and WIEN2k. The CASTEP calculations of δiso are generally in very good agreement with experiment, having r.m.s. deviation 0.40 ppm. WIEN2k calculations of electric field gradient magnitude, CQ, and asymmetry, η, are also in excellent agreement with experiment, with r.m.s. deviations 0.038 MHz and 0.042 respectively. However, whilst CASTEP gives a similar deviation for η (0.043) it overestimates CQ by ∼15%. After scaling of the calculated electric field gradient by 0.842 the deviation in CQ is practically identical to that of the WIEN2k calculations. The spectral assignments that follow from the experimental and computational results allow identification of correlations between δiso and (a) the average B-O-B bond angle, θ[combining overline], for both three and four coordinated boron, giving δiso(B(III)) = (185.1 -θ[combining overline])/3.42 ppm and δiso(B(IV)) = (130.2 -θ[combining overline])/5.31 ppm; and (b) the ring-site T(3) unit trigonal planar angular deviation, Stri, giving δiso(T(3)(ring)) = (1.642 × 10(-2)-Stri)/(8.339 × 10(-4)) ppm.