X-ray and neutron scattering measurements of ordering in a Cu46Zr54 liquid.

The structural evolution of the equilibrium and supercooled Cu46Zr54 liquids was investigated with a combination of elastic neutron scattering (with isotopic substitution) and synchrotron x-ray scattering studies. The partial pair correlation functions were determined over a wide temperature range (∼270 °C). These show that the Cu-Cu and Zr-Zr ordering increases as the temperature decreases, while the Cu-Zr ordering decreases. This surprising result is in contradiction with the results from molecular dynamics studies.

Incommensurate Magnetism Near Quantum Criticality in CeNiAsO.

We report the discovery of incommensurate magnetism near quantum criticality in CeNiAsO through neutron scattering and zero field muon spin rotation. For T

Molten barium titanate: a high-pressure liquid silicate analogue.

The structure of molten BaTiO3 has been measured using laser heating, aerodynamic levitation and a combination of neutron diffraction with Ti isotope substitution, x-ray diffraction and spectroscopy. All measurements indicate a Ti-O coordination of n TiO = 4.4(2), far lower than the perovskite or hexagonal crystalline forms. However, n TiO > 4 suggests structural analogy with molten silicates at high pressures. We introduce methodology for ascertaining such analogies and demonstrate similarity with molten CaSiO3 at upper mantle pressures circa 5 GPa. Although some topological differences exist, we propose that planetary melt analogues provide rich insight into important processes relevant to hot exoplanets and Earth's early history.

A suite-level review of the neutron powder diffraction instruments at Oak Ridge National Laboratory.

The suite of neutron powder diffractometers at Oak Ridge National Laboratory (ORNL) utilizes the distinct characteristics of the Spallation Neutron Source and High Flux Isotope Reactor to enable the measurements of powder samples over an unparalleled regime at a single laboratory. Full refinements over large Q ranges, total scattering methods, fast measurements under changing conditions, and a wide array of sample environments are available. This article provides a brief overview of each powder instrument at ORNL and details the complementarity across the suite. Future directions for the powder suite, including upgrades and new instruments, are also discussed.

Favorable Concurrence of Static and Dynamic Phenomena at the Morphotropic Phase Boundary of xBiNi_{0.5}Zr_{0.5}O_{3}-(1-x)PbTiO_{3}.

We reveal that concurrent events of inherent entropy boosting and increased synchronization between A- and B-site cation vibrations of an ABO_{3}-type perovskite structure give rise to a larger piezoelectric response in a ferroelectric system at its morphotropic phase boundary (MPB). It is further evident that the superior piezoelectric properties of xBiNi_{0.5}Zr_{0.5}O_{3}-(1-x)PbTiO_{3} in comparison to xBiNi_{0.5}Ti_{0.5}O_{3}-(1-x)PbTiO_{3} are due to the absolute flattening of the local potentials for all ferroelectrically active cations with a higher spontaneous polarization at the MPB. These distinctive features are discovered from the analyses of neutron pair distribution functions and Raman scattering data at ambient conditions, which are particularly sensitive to mesoscopic-scale structural correlations. Altogether this uncovers more fundamental structure-property connections for ferroelectric systems exhibiting a MPB, and thereby has a critical impact in contriving efficient novel materials.

Local Structure and Short-Range Order in a NiCoCr Solid Solution Alloy.

Multielement solid solution alloys are intrinsically disordered on the atomic scale, and many of their advanced properties originate from the local structural characteristics. The local structure of a NiCoCr solid solution alloy is measured with x-ray or neutron total scattering and extended x-ray absorption fine structure (EXAFS) techniques. The atomic pair distribution function analysis does not exhibit an observable structural distortion. However, an EXAFS analysis suggests that the Cr atoms are favorably bonded with Ni and Co in the solid solution alloys. This short-range order (SRO) may make an important contribution to the low values of the electrical and thermal conductivities of the Cr-alloyed solid solutions. In addition, an EXAFS analysis of Ni ion irradiated samples reveals that the degree of SRO in NiCoCr alloys is enhanced after irradiation.

Mercury Sulfide Dimorphism in Thioarsenate Glasses.

Crystalline mercury sulfide exists in two drastically different polymorphic forms in different domains of the P,T-diagram: red chain-like insulator α-HgS, stable below 344 °C, and black tetrahedral narrow-band semiconductor ß-HgS, stable at higher temperatures. Using pulsed neutron and high-energy X-ray diffraction, we show that these two mercury bonding patterns are present simultaneously in mercury thioarsenate glasses HgS-As2S3. The population and interconnectivity of chain-like and tetrahedral dimorphous forms determine both the structural features and fundamental glass properties (thermal, electronic, etc.). DFT simulations of mercury species and RMC modeling of high-resolution diffraction data provide additional details on local Hg environment and connectivity implying the (HgS2/2)m oligomeric chains (1 ≤ m ≤ 6) are acting as a network former while the HgS4/4-related mixed agglomerated units behave as a modifier.

Electrostatic levitation facility optimized for neutron diffraction studies of high temperature liquids at a spallation neutron source.

Neutron diffraction studies of metallic liquids provide valuable information about inherent topological and chemical ordering on multiple length scales as well as insight into dynamical processes at the level of a few atoms. However, there exist very few facilities in the world that allow such studies to be made of reactive metallic liquids in a containerless environment, and these are designed for use at reactor-based neutron sources. We present an electrostatic levitation facility, NESL (for Neutron ElectroStatic Levitator), which takes advantage of the enhanced capabilities and increased neutron flux available at spallation neutron sources (SNSs). NESL enables high quality elastic and inelastic neutron scattering experiments to be made of reactive metallic and other liquids in the equilibrium and supercooled temperature regime. The apparatus is comprised of a high vacuum chamber, external and internal neutron collimation optics, and a sample exchange mechanism that allows up to 30 samples to be processed between chamber openings. Two heating lasers allow excellent sample temperature homogeneity, even for samples approaching 500 mg, and an automated temperature control system allows isothermal measurements to be conducted for times approaching 2 h in the liquid state, with variations in the average sample temperature of less than 0.5%. To demonstrate the capabilities of the facility for elastic scattering studies of liquids, a high quality total structure factor for Zr64Ni36 measured slightly above the liquidus temperature is presented from experiments conducted on the nanoscale-ordered materials diffractometer (NOMAD) beam line at the SNS after only 30 min of acquisition time for a small sample (∼100 mg).

Note: Detector collimators for the nanoscale ordered materials diffractometer instrument at the Spallation Neutron Source.

Five neutron collimator designs were constructed and tested at the nanoscale ordered materials diffractometer (NOMAD) instrument. Collimators were made from High Density PolyEthylene (HDPE) or 5% borated HDPE. In all cases, collimators improved the signal to background ratio and reduced detection of secondary scattering. In the Q-range 10-20 Å(-1), signal to background ratio improved by factors of approximately 1.6 and 2.0 for 50 and 100 mm deep collimators, respectively. In the Q-range 40-50 Å(-1), the improvement factors were 1.8 and 2.7. Secondary scattering as measured at Q ∼ 9.5 Å(-1) was significantly decreased when the collimators were installed.

The structure of water around the compressibility minimum.

Here we present diffraction data that yield the oxygen-oxygen pair distribution function, g(OO)(r) over the range 254.2-365.9 K. The running O-O coordination number, which represents the integral of the pair distribution function as a function of radial distance, is found to exhibit an isosbestic point at 3.30(5) Å. The probability of finding an oxygen atom surrounding another oxygen at this distance is therefore shown to be independent of temperature and corresponds to an O-O coordination number of 4.3(2). Moreover, the experimental data also show a continuous transition associated with the second peak position in g(OO)(r) concomitant with the compressibility minimum at 319 K.

Low cation coordination in oxide melts.

The complete set of partial pair distribution functions for a rare earth oxide liquid are measured by combining aerodynamic levitation, neutron and x-ray diffraction on Y2O3, and Ho2O3 melts at 2870 K. The average Y-O (or Ho-O) coordination of these isomorphic melts is measured to be 5.5(2), which is significantly less than the octahedral coordination of crystalline Y2O3 (or Ho2O3). Investigation of La2O3, ZrO2, and Al2O3 melts by x-ray diffraction and molecular dynamics simulations also show lower-than-crystal cation-oxygen coordination. These measurements suggest a general trend towards lower coordination compared to their crystalline counterparts. It is found that the coordination drop is larger for lower field strength, larger radius cations and is negligible for high field strength (network forming) cations, such as SiO2. These findings have broad implications for predicting the local structure and related physical properties of metal-oxide melts and oxide glasses.

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).

Acoustic levitator for structure measurements on low temperature liquid droplets.

A single-axis acoustic levitator was constructed and used to levitate liquid and solid drops of 1-3 mm in diameter at temperatures in the range -40 to +40 degrees C. The levitator comprised (i) two acoustic transducers mounted on a rigid vertical support that was bolted to an optical breadboard, (ii) an acoustic power supply that controlled acoustic intensity, relative phase of the drive to the transducers, and could modulate the acoustic forces at frequencies up to 1 kHz, (iii) a video camera, and (iv) a system for providing a stream of controlled temperature gas flow over the sample. The acoustic transducers were operated at their resonant frequency of approximately 22 kHz and could produce sound pressure levels of up to 160 dB. The force applied by the acoustic field could be modulated to excite oscillations in the sample. Sample temperature was controlled using a modified Cryostream Plus and measured using thermocouples and an infrared thermal imager. The levitator was installed at x-ray beamline 11 ID-C at the Advanced Photon Source and used to investigate the structure of supercooled liquids.

The structure of liquid carbon dioxide and carbon disulfide.

We present neutron and x-ray scattering data (a 2N+X experiment) of liquid CO(2) and CS(2) at a density of about 10 molecules/nm(3). Because the scattering length contrast of the carbon isotope is very small and, in fact, smaller than anticipated from standard scattering length tables, a direct partial structure factor determination via matrix inversion gives unconvincing results. Instead we search for the best representation of the three independent scattering data sets by a simulation of rigid molecules interacting via a 12-6-1 potential, furthermore restricting the pressure p, the density rho, and the temperature T to the experimental values. We show that a 12-6-1 potential is completely adequate to describe the structure of CO(2); for CS(2) we find that the best 12-6-1 potential still slightly overestimates the height of the sulfur-sulfur pair-distribution function g(SS). Orientational correlations reflect the similarities much more than the differences of the two molecular systems. The distinct differences in the atom-atom pair distribution functions of CO(2) and CS(2) do not mean that their structures are radically different and the comparison with the crystalline structures is somewhat deceptive. A linear transformation, wherein all the parameters describing the interaction and the geometry of CS(2) are changed to those of CO(2), allows us to point out the physical parameters which may be responsible for the differences or similarities in thermodynamic behavior (pressure) and structures (orientations) between the two liquids.

Adding a length scale to the polyamorphic ice debate.

X-ray scattering and molecular dynamics simulations have been used to correlate the short range oxygen-oxygen structure with the intermediate range ordering (IRO) upon annealing very high density amorphous ice. While it is clear that the IRO that defines the network structure breaks down continuously to a minimum level, where there are weakened correlations extending beyond 7 Angstrom, at this point the local structure (O-O-O angles) is observed to change abruptly, allowing a continuous reemergence of a new IRO network. This is very different from a classic first order transition and helps reconcile previous data.

Isotope quantum effects in water around the freezing point.

We have measured the difference in electronic structure factors between liquid H(2)O and D(2)O at temperatures of 268 and 273 K with high energy x-ray diffraction. These are compared to our previously published data measured from 279 to 318 K. We find that the total structural isotope effect increases by a factor of 3.5 over the entire range, as the temperature is decreased. Structural isochoric temperature differential and isothermal density differential functions have been used to compare these data to a thermodynamic model based upon a simple offset in the state function. The model works well in describing the magnitude of the structural differences above approximately 310 K, but fails at lower temperatures. The experimental results are discussed in light of several quantum molecular dynamics simulations and are in good qualitative agreement with recent temperature dependent, rotationally quantized rigid molecule simulations.

Determination of actinide speciation in solution using high-energy X-ray scattering.

High-energy X-ray scattering (HEXS) has been used to understand the coordination environment of the uranyl ion in a perchlorate solution. Assuming the two coordinating oxo ligands bound to U(VI) are represented in a peak in the pair distribution function (PDF) at 1.766(1) A, integration of the peak intensity is used to quantify the charge located on the oxygens. The dioxo ligands are essentially neutral, as predicted by numerous published calculations, with a charge of -16.4(8) electrons. The peak in the PDF at 2.420(1) A is consistent with equatorial ligating waters. The intensity of this peak is inconsistent with an integral coordination number and is used to propose a solution equilibrium of five and four waters coordinating to the uranyl(VI) ion favoring the five-coordinate species. This equilibrium is then used to experimentally determine that five-coordinate uranyl is 1.19+/-0.42 kcal/mol more stable than its four-coordinate counterpart under the conditions of the experiment. Further peaks in the Fourier transform of the scattering data at 4.50, 7, and 8.7 A are attributed to uranium-solvent correlations.

Temperature dependence of isotopic quantum effects in water.

The technique of high energy x-ray diffraction has been used to measure the temperature variation of hydrogen versus deuterium isotopic quantum effects on the structure of water. The magnitude of the effect is found to be inversely proportional to the temperature, varying by a factor of 2.5 over the range 6 to 45 degrees C. In addition, the H216O versus H218O effect has been measured at 26 degrees C and the structural difference shown to be restricted to the nearest neighbor molecular interactions. The results are compared to recent simulations and previously measured isochoric temperature differentials; additionally, implications for H/D substitution experiments are considered.

Structure of the UO2 2+-SO 4 2- ion pair in aqueous solution.

The structure of the ion pairs formed in aqueous uranyl sulfate solutions has been investigated with high-energy X-ray scattering. Sulfate binds to the uranyl as a monodentate ligand in equimolar solutions. The geometry of the ion pair is very similar to configurations found in crystalline structures; in particular, the U-O-S angle is bent in solution as well as in the solid state. It can therefore be concluded that an U-O-S angle of 143 degrees is an intrinsic property of the uranyl sulfate bond and not due to packing effects or interaction with the water in the primary solvation shell.