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Neuefeind, Joerg C

Publications and source records attributed to Neuefeind, Joerg C.

17 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Polymorphism in Photoluminescent KNdW2O8: Synthesis, Neutron Diffraction, and Raman Study

Polymorphs of KNdW2O8 ( -KNdW2O8 and -KNdW2O8) phosphors were synthesized by an efficient solution combustion technique for the first time. The crystal structure of the polymorphs analyzed from Rietveld refinement of neutron diffraction data confirms that -KNdW2O8 crystallizes in the tetragonal system (space group I4 ), and -KNdW2O8 crystallizes in the monoclinic system (space group C2/m). The local structure of both polymorphs was elucidated using combined neutron pair distribution function (PDF) and Raman scattering techniques. Photoluminescence measurements of the polymorphs showed broadened emission line width and increased intensity for -KNdW2O8 in the visible region compared to -KNdW2O8. This phenomenon is attributed to the increased distortion in the coordination environment of the luminescing Nd3+ ion. Combined PDF, Rietveld, and Raman measurements reveal distortions of the WO6 octahedra and NdO8 polyhedra in -KNdW2O8. This crystal structure photoluminescence study suggests that this class of tungstates can be exploited for visible light emitting devices by tuning the crystal symmetry.

Swain, Diptikanta↗
DOE OSTI2013

Measurements of liquid and glass structures using in-situ high energy x-ray and neutron scattering

Investigation of high temperature molten materials and their evolution to the amorphous state is often hampered by unwanted reactions with container surfaces. This work used aerodynamic levitation in combination with laser beam heating to study high melting point materials that can form supercooled liquids or glasses. Details of the instruments that are being used at the Advanced Photon Source and the Spallation Neutron Source to study molten oxides with high energy x-ray scattering and neutron diffraction with isotope substitution are presented. Examples of measurements are used to illustrate the use of the instruments. Plans for further development and application of the capabilities are presented.

Weber, Richard↗
DOE OSTI2013

Crystal and magnetic structures and physical properties of a new pyroxene NaMnGe2O6 synthesized under high pressure

A new pyroxene NaMnGe2O6 has been synthesized at 3 GPa and 800 C, and fully characterized by x-ray single-crystal diffraction and neutron powder diffraction, measurements of magnetization and specific heat. Like other majority sodium pyroxenes, NaMnGe2O6 crystallizes into a monoclinic C2/c structure with unit-cell parameters a = 9.859(2) , b = 8.7507(18) , c = 5.5724(11) , and =105.64(3) at room temperature. The crystal structure is featured by quasi-one-dimensional chains of skew edge-sharing MnO6 octahedra running along the crystallographic c axis; these chains are connected by non-magnetic GeO4 tetrahedra, so as to lead to a low-dimensional magnetism. The highly distorted MnO6 octahedron consisting of three Mn-O bond lengths, i.e. 1.918 , 1.991 , and 2.198 , is consistent with the Jahn-Teller effect at Mn3+ in a cubic crystal field. A long-range cooperative Jahn-Teller distortion is formed by ordering longest Mn-O bonds between two neighboring octahedra along the chain direction. No orbital order-disorder transition has been found up to 750 K as checked by magnetic susceptibility. Like other alkali-metal pyroxenes with S > , NaMnGe2O6 (S = 2) was found to undergo a long-range antiferromagnetic ordering at TN = 7 K at low magnetic field due to the exchange interactions along and between chains. Due to the peculiar structural features and the corresponding magnetic coupling, the weak AF spin ordering gives way to a ferromagnetic-like state at a sufficiently high magnetic field. Specific-heat measurements demonstrated that a large portion of the magnetic entropy, i.e. > 60 %, has been removed above TN as a result of strong spin correlations within the quasi-one-dimensional Mn3+-spin chains. Neutron powder diffraction study suggests a commensurate magnetic structure defined by k = [0 0 0.5] with Mn moments aligned along the c axis. The present study on NaMnGe2O6 completed the evolution of magnetic properties as a function of the d-orbital occupancy from d1 to d5 in the magnetic pyroxenes.

Yan, Jiaqiang↗
DOE OSTI2013

Local atomic structural investigations of precursory phenomenon of the hydrogen release from LiAlD4

Local atomic structural investigations of LiAlD4, which is composed of Li+ and [AlD4], at 40 300 K were studied by total neutron scattering combined with pair distribution function (PDF) analysis for understanding of hydrogen release from LiAlD4. The results showed that the Al D pair distribution almost unchanged, while the Li D pair distribution clearly started to broaden and shrink above 200 250 K. The shrinking of the Li D pair distribution might lead to the local generation of LiD, which was speculated as the precursory phenomenon for the hydrogen release from LiAlD4.

Sato, Toyoto↗
DOE OSTI2012

A green-yellow emitting oxyfluoride solid solution phosphor Sr[subscript 2]Ba(AlO[subscript 4]F)[subscript 1;#8722;x](SiO[subscript 5])x:Ce[superscript 3+] for thermally stable, high color rendition solid state white lighting

A near-UV excited, oxyfluoride phosphor solid solution Sr{sub 1.975}Ce{sub 0.025}Ba(AlO{sub 4}F){sub 1-x}(SiO{sub 5}){sub x} has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce{sup 3+} activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green ({lambda}{sub em} = 523 nm) to yellow ({lambda}{sub em} = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 {+-} 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED ({lambda}{sub max} = 400 nm) exhibit high color rendering white light with R{sub a} = 70 and a correlated color temperature near 7000 K. The value of R{sub a} could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Isotope effects in water as investigated by neutron diffraction and path integral molecular dynamics

The structure of heavy and light water at 300 K was investigated by using a joint approach in which the method of neutron di raction with oxygen isotope substitution was combined with path integral molecular dynamics simulations. The di raction results, which give intra-molecular O-D and O-H bond distances of 0.985(5) and 0.990(5) A, were found to be in best agreement with those obtained by using the exible anharmonic TTM3-F water model. Both techniques show a di erence of '0.5% between the O-D and O-H intra-molecular bond lengths and the results support a competing quantum e ects model for water in which its structural and dynamical properties are governed by an o set between intra-molecular and inter-molecular quantum contributions. Further consideration of the O-O correlations is needed in order to improve agreement with experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

The bound coherent neutron scattering length of the oxygen isotopes

The technique of neutron interferometry was used to measure the bound coherent neutron scattering length bcoh of the oxygen isotopes 17O and 18O. From the measured difference in optical path between two water samples, either H2 17O or H2 18O versus H2 natO, where nat denotes the natural isotopic composition, we obtain bcoh , 17O = 5.867(4) fm and bcoh , 18O = 6.009(5) fm, based on the accurately known value of bcoh , natO = 5.805(4) fm which is equal to bcoh , 16O within the experimental uncertainty. Our results for bcoh , 17O and bcoh , 18O differ appreciably from the standard tabulated values of 5.6(5) fm and 5.84(7) fm, respectively. In particular, our measured scattering length contrast of 0.204(3) fm between 18O and natO is nearly a factor of 6 greater than the tabulated value, which renders feasible neutron diffraction experiments using 18O isotope substitution and thereby offers new possibilites for measuring the partial structure factors of oxygen-containing compounds, such as water.

Fischer, Henry E↗
DOE OSTI2012

The Nanoscale Ordered MAterials Diffractometer NOMAD at the Spallation Neutron Source SNS

The Nanoscale Ordered Materials Diffractometer (NOMAD) is neutron time-of-flight diffractometer designed to determine pair dist ribution functions of a wide range of materials ranging from short range ordered liquids to long range ordered crystals. Due to a large neutron flux provided by the Spallation Neutron Source SNS and a large detector coverage neutron count-rates exceed comparable instruments by one to two orders of magnitude. This is achieved while maintaining a relatively high momentum transfer resolution of a $\delta Q/Q \sim 0.8\%$ FWHM (typical), and an achievable $\delta Q/Q$ of 0.24\% FWHM (best). The real space resolution is related to the maximum momentum transfer; A maximum momentum transfer of 50\AA$^{-1}$ can be achieved routinely and the maximum momentum transfer given by the detector configuration and the incident neutron spectrum is 125 \AA$^{-1}$. High stability of the source and the detector allow small contrast isotope experiments to be performed. A detailed description of the instrument is given and the results of experiments with standard samples are discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Oxygen as a site specific structural probe in neutron diffraction

Oxygen is a ubiquitous element, playing an essential role in most scientific and technological disciplines, and is often incorporated within a structurally disordered material where examples include molten silicates in planetary science, glasses used for lasers and optical communication, and water in biological processes. Establishing the structure of a liquid or glassy oxide and thereby its relation to the functional properties of a material is not, however, a trivial task owing to the complexity associated with atomic disorder. Here we approach this challenge by measuring the bound coherent neutron scattering lengths of the oxygen isotopes with the sensitive technique of neutron interferometry. We find that there is a small but finite contrast of 0.204(6) fm between the scattering lengths of the isotope 18O and oxygen of natural isotopic abundance natO, contrary to tables of recommended values. This has enabled us to investigate the structure of both light and heavy water by exploiting, for the first time, the method of oxygen isotope substitution in neutron diffraction, thus circumventing many of the significant problems associated with more traditional methods in which hydrogen is substituted by deuterium. We find a difference of ~0.5% between the O-H and O-D intra-molecular bond distances which is much smaller than recent estimates based on diffraction data and is found to be in excellent agreement with path integral molecular dynamics simulations made with a flexible polarisable water model. Our results demonstrate the potential for using oxygen isotope substitution as a powerful and effective site specific probe in a plethora of materials, of pertinence as instrumentation at next generation neutron sources comes online

08 HYDROGEN↗
DOE OSTI2011

Instrumentation for Structure Measurements on Highly Non-equilibrium Materials

Containerless techniques (levitation) completely eliminate contact with the sample. This unique sample environment allows deep supercooling of many liquids and avoids contamination of high temperature melts. Recent experiments at the APS high energy beamline 11 ID-C used aerodynamic levitation with laser beam heating and acoustic levitation with cryogenic cooling. By using these two methods, liquids were studied over much of the temperature range from -40 to +2500 C. This paper briefly describes the instrumentation and its use with an -Si area detector that allows fast, in-situ measurements. Use of the instruments is illustrated with examples of measurements on molten oxides and aqueous materials.

Weber, Richard↗