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J Neuefeind

Publications and source records attributed to J Neuefeind.

8 recordsLinked to original sources
PubMed2006

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.

Journal Article↗
PubMed2005

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.

Actinoid Series Elements↗
PubMed2005

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.

Computer Simulation↗
PubMed2004

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.

Journal Article↗
PubMed2003

Intermediate-range order in permanently densified GeO2 glass.

Information about the partial structure factors of densified GeO2 glass has been obtained from neutron and x-ray diffraction measurements. Densification causes a reduction in the length scale of the intermediate range order (IRO). The difference structure factors obtained by combining the x-ray and neutron data so as to eliminate one partial structure factor at a time shows the greatest effects when the Ge-Ge correlations are eliminated and least when O-O correlations are eliminated. This implies that the reduced length scale results from a decrease in the next-nearest neighbor Ge-O and O-O distance caused by a rotation about the Ge-O-Ge bonds and a distortion of the GeO4 tetrahedra.

Journal Article↗
PubMed2002

Structural studies of several distinct metastable forms of amorphous ice.

Structural changes during annealing of high-density amorphous ice were studied with both neutron and x-ray diffraction. The first diffraction peak was followed from the high- to the low-density amorphous form. Changes were observed to occur through a series of intermediate forms that appear to be metastable at each anneal temperature. Five distinct amorphous forms were studied with neutron scattering, and many more forms may be possible. Radial distribution functions indicate that the structure evolves systematically between 4 and 8 angstroms. The phase transformations in low-temperature liquid water may be much more complex than currently understood.

Journal Article↗
PubMed1998

A triple-crystal diffractometer for high-energy synchrotron radiation at the HASYLAB high-field wiggler beamline BW5.

The triple-crystal diffractometer installed at HASYLAB beamline BW5 with a high-field wiggler of critical energy 27 keV for DORIS III, operated at 4.5 GeV electron energy, is described. Samples can be mounted in large cryostats or furnaces normally used in neutron scattering experiments. The instrument has been successfully applied to measure structure factors S(Q) in liquids and amorphous materials, to collect full data sets of highly accurate structure factors for charge-density work, to measure the spin component of the ground-state magnetization in transition-metal and rare-earth compounds, to study the diffuse scattering from stacking faults and dislocation loops in Si single crystals, and for the investigation of various aspects of structural phase transitions: critical scattering in SrTiO3, oxygen order and stripe order in high-To materials. A crossed-beam technique allows for local studies of texture, internal strain and phase changes in the bulk of materials.

Journal Article↗