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Neutron diffraction studies of high-T/sub c/ superconductors

Description: Neutron powder diffraction techniques have been used extensively for the study of high-T/sub c/ oxide superconductors because of the need to locate oxygen atoms and accurately determine the oxygen site occupancies, and the difficulty in obtaining single crystals. For example, in the case of YBa/sub 2/Cu/sub 3/O/sub 7-delta/, neutron powder diffraction and Rietveld structural refinement were used to obtain the first complete structural information. Subsequent experiments focussed on determining the relationship of superconducting properties to the number and distribution of oxygen vacancies on the Cu-O sublattice with measurements being done on samples in thermodynamic equilibrium, at high temperature in controlled oxygen atmospheres, and on metastable, oxygen-deficient samples produced by quenching. Neutron powder diffraction has also been used to determine the structures of compounds in which the properties have been modified by substitution on the Y, Ba, or Cu sites. This paper briefly reviews some of the neutron powder diffraction results in these areas. 17 refs.
Date: March 1, 1988
Creator: Jorgensen, J.D.
Partner: UNT Libraries Government Documents Department

Electronically focused power diffractometers at IPNS-I

Description: Two powder diffractometers have been operated at IPNS-I since August 1981. The diffractometers achieve high resolution with large detector solid angles for scattering angles from +- 12 to 157/sup 0/ by electronically focussing the events from individual detectors in an on-line microprocessor.
Date: November 1, 1982
Creator: Jorgensen, J.D. & Faber, J.
Partner: UNT Libraries Government Documents Department

Key insights from structural studies of high-temperature superconductors : Is there a path to higher Tc?

Description: Structural studies have allowed the development of a model for the ''ideal'' high-temperature superconductor. For a given compound, the maximum T{sub c} is traditionally achieved by using a chemical variable to adjust the carrier concentration to the optimum value. When comparing different compounds at their optimum doping, the highest T{sub c} is observed for compounds with flat CuO{sub 2} planes. T{sub c} can also be enhanced if the charge reservoir region, or blocking layer, is metallic. In general, these three criteria cannot simultaneously be met by adjusting a single chemical/structural variable. Additionally, recent work on HgBa{sub 2}CuO{sub 4+x} and 123 compounds as a function of doping suggest that electronically-driven structural distortions may hinder attempts to produce higher T{sub c}'s by chemical substitutions. In spite of these challenges, the ideal high-T{sub c} compound has not yet been discovered and the search should continue.
Date: October 1, 1999
Creator: Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Novel structural phenomena at the maximum T{sub c} in 123 and HgBa{sub 2}CuO{sub 4+{delta}} superconductors : evidence for a structural response that competes with superconductivity.

Description: Structural distortions that compete with superconductivity have been investigated in two systems where oxygen content can be used to vary the doping continuously from the under doped state, through the maximum T{sub c} into the over doped state. In the 123 system, (La{sub 1{minus}x}Ca{sub x})(Ba{sub 1.75{minus}x}La{sub 0.25+x})Cu{sub 3}O{sub 7+{delta}}, the buckling of the CuO{sub 2} planes goes through a maximum at the maximum T{sub c}. In HgBa{sub 2}CuO{sub 4+{delta}}, where buckling of the CuO{sub 2} planes is not available as a structural degree of freedom, there is a plateau at the maximum T{sub c} where the unit cell volume expands as oxygen is added while the charge transfer and T{sub c} remain constant. These unusual structural phenomena upon crossing through the maximum T{sub c} are hypothesized to be a response of the crystal structure to the electronic structure, with the structural distortions competing with superconductivity, or lowering the T{sub c} from what it would otherwise be.
Date: June 3, 1998
Creator: Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Time-of-flight diffraction at pulsed neutron sources: An introduction to the symposium

Description: In the 25 years since the first low-power demonstration experiments, pulsed neutron sources have become as productive as reactor sources for many types of diffraction experiments. The pulsed neutron sources presently operating in the United States, England, and Japan offer state of the art instruments for powder and single crystal diffraction, small angle scattering, and such specialized techniques as grazing-incidence neutron reflection, as well as quasielastic and inelastic scattering. In this symposium, speakers review the latest advances in diffraction instrumentation for pulsed neutron sources and give examples of some of the important science presently being done. In this introduction to the symposium, I briefly define the basic principles of pulsed neutron sources, review their development, comment in general terms on the development of time-of-flight diffraction instrumentation for these sources, and project how this field will develop in the next ten years.
Date: May 1, 1994
Creator: Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Structural phase transition in CeCu/sub 6/

Description: The structure of CeCu/sub 6/ has been studied at eight temperatures from 10K to 295K by time-of-flight neutron powder diffraction. The high temperature orthorhombic cell transforms to a monoclinic cell at 230K. Rietveld structural refinements at each temperature give details of the structural distortion associated with the transition. The space groups and the temperature dependence of the monoclinic strain are consistent with a second order transition driven by a soft acoustic phonon mode. However, small atom displacements perpendicular to the shear direction suggest that an optic mode may also be involved in the transition.
Date: August 1, 1985
Creator: Vrtis, M.L.; Jorgensen, J.D. & Hinks, D.G.
Partner: UNT Libraries Government Documents Department

Pressure induced strain transition in NiF/sub 2/

Description: Time-of-flight neutron powder diffraction measurements are reported which give the first direct evidence that NiF/sub 2/ undergoes a pressure induced, continuous strain transition from a tetragonal P4/sub 2//mnm (rutile) to an orthorhombic Pnnm structure at 1.83 +- 0.1 GPa. Lattice, structural and thermal parameters have been determined at pressures to 3.24 GPa. Even though the orthorhombic strain is sufficiently small that doublets are not resolved, the use of profile refinement of the diffraction spectra allows us to accurately determine the lattice parameters near the transition and to show that (b - a)/sup 2/ versus pressure is a straight line with slope 2.27 x 10/sup -3/A/sup 2//GPa going to zero at the transition pressure. The measured atomic positions and anisotropic thermal displacements show that motion of the F atoms is responsible for the transition. All of our findings are consistent with a second order transition involving the softening of a transverse acoustic phonon mode propagating along <110> and polarized along <1 anti 10>.
Date: January 1, 1977
Creator: Jorgensen, J. D.; Worlton, T. G. & Jamieson, J. C.
Partner: UNT Libraries Government Documents Department

Rietveld neutron powder profile analysis and electrical conductivity of the fast silver-ion conductor (LaO)AgS

Description: Lanthanum silver oxysulfide, (LaO)AgS, exhibits a predominantly ionic conductivity of 10{sup {minus}3} to 10{sup {minus}1} S/cm between 300 K and 770 K. The tetragonal structure consists of alternating (LaO) and (AgS) sheets, their sequence being O-La-S-Ag-S-La-O. The structure suggests that ionic transport arises from migration of silver ions within the AgS layers analogous to sodium ion transport in Na-{beta}-alumina. Neutron powder diffraction data measured at five temperatures between 300 K and 770 K are analyzed using the Rietveld method to determine the distribution and thermal vibration parameters of the mobile silver ions. The structural investigation is accompanied by measurements of the total conductivity in the same temperature range in order to resolve severe discrepancies in the literature data.
Date: November 18, 1999
Creator: Wilmer, D.; Wuensch, B. J. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Structure--properties relationships for manganese perovskites

Description: By combining the results of dc magnetization, ac susceptibility, magnetoresistivity, magnetostriction, and x-ray and neutron powder diffraction data for stoichiometric La{sub 1{minus}x}Sr{sub x}MnO{sub 3} the authors have constructed a phase diagram that describes the magnetic, transport, and structural properties and the relationships among them as a function of composition and temperature. Correlations among physical and structural properties have been observed that are consistent with a competition between ferromagnetism and JT distortion. A metallic state occurs below the Curie temperature when both coherent and incoherent JT distortions are suppressed.
Date: October 14, 1999
Creator: Dabrowski, B.; Xiong, X.; Chmaissem, O.; Bukowski, Z. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Rare-gas intercalation into fullerene interstices

Description: The authors review their neutron diffraction studies of the intercalation of the rare gases into the interstices of C{sub 60} at pressures ranging from 0 to 6 kbar, primarily at room temperature, but also at other temperatures. Room temperature compressibilities of C{sub 60} for both the face-centered-cubic and simple-cubic phases using Ar, which does not intercalate, as the pressure medium are in excellent agreement with the earlier work of David et al, while the values for He and Ne are slightly smaller than those using Ar. The rates of intercalation and release of Ne, at a number of different temperatures, have been measured and show that, at a given temperature, intercalation is much slower than release. Structural refinements of the Ne intercalated C{sub 60} have been carried out for room temperature samples at many different pressures. These data suggest that Ne only intercalates into the octahedral hole and that the occupancy increases smoothly with pressure. Intercalated materials are important technologically for their use as catalysts, waste storage media, solid-state batteries, molecular sieves, gettering materials, and in many other applications. Sometimes, intercalation endows the new material with remarkable properties.
Date: August 1, 1995
Creator: Kwei, G.H.; Jorgensen, J.D.; Schirber, J.E. & Morosin, B.
Partner: UNT Libraries Government Documents Department

Pulsed-neutron techniques for condensed-matter research

Description: Pulsed spallation sources are reviewed in a historical content as the latest generation of neutron sources in a line that started with the discovery of the neutron in 1932 and proceeded through research-reactor and accelerator-driven sources. The characteristics of the spallation sources are discussed in relation to their capabilities for structural and dynamical studies of condensed matter with slow neutrons and radiation effects research with fast neutrons. The new scientific opportunities opened up in these fields by the unique features of the sources are briefly reviewed, with some examples of completed work and experiments being planned.
Date: January 1, 1981
Creator: Brown, B.S.; Carpenter, J.M.; Jorgensen, J.D.; Price, D.L. & Kamitakahara, W.
Partner: UNT Libraries Government Documents Department

Mossbauer effect and neutron diffraction study of the anomalous lattice behavior of U/sub 6/Fe

Description: Neutron diffraction and /sup 57/Fe Moessbauer effect measurements have been made on the exchange-enhanced superconductor U/sub 6/Fe. The neutron diffraction results indicate a pronounced anisotropy in the thermal expansion of the unit cell and an anomalous temperature dependence for the c-axis component of the total Debye-Waller factor of Fe below 100K. The Moessbauer effect measurements show non-Debye behavior in the mean-squared displacement of Fe, with phonon hardening occurring below 100K. The absence of a temperature dependence to the relative intensities of the lines in the Fe quadrupole doublet indicates almost isotropic lattice vibrations. Taken together these observations imply that Fe undergoes static displacements along the c-axis that begin below 100K and increase with decreasing temperature. Both the Moessbauer shift and the quadrupole coupling indicate electronic changes in the valence of Fe in the temperature region in which the displacement takes place.
Date: June 1, 1985
Creator: Vaishnava, P.P.; Dwight, A.E.; Kimball, C.W.; Jorgensen, J.D. & Fradin, F.Y.
Partner: UNT Libraries Government Documents Department

Structure of high-pressure KNO/sub 3/-IV

Description: Potassium nitrate has seven polymorphs in the pressure range to 40 kbar. The structures of the three phases obtainable at atmospheric pressure are known. Attempts to solve the structure of high pressure KNO/sub 3/ phase IV by x-ray diffraction have been unsuccessful. We have now solved this structure using time-of-flight neutron powder diffraction data taken at room temperature and 3.6 kbar on the SEPD at IPNS. High pressure KNO/sub 3/-IV has the same space group and point group symmetry as the low pressure KNO/sub 3/-II, but with different values for lattice parameters and atomic positions. The structure of KNO/sub 3/-IV is orthorhombic Pnma with a = 7.4867(2), b = 5.5648(2), and c = 6.7629(2) with four formula units per unit cell. The K, N, and one O atom are in special positions (4c) and the other O atoms are in the general position (8d) just as in the atmospheric pressure phase. The O-N-O bond angles are 120.4(1)/sup 0/, and 119.3(2)/sup 0/. The N-O bond lengths are 1.228(3)A and 1.246(2)A.
Date: July 1, 1985
Creator: Worlton, T.G.; Decker, D.L.; Jorgensen, J.D. & Kleb, R.
Partner: UNT Libraries Government Documents Department

Structural phase transitions in BaMo/sub 6/S/sub 8/: Evidence for an incommensurate phase

Description: The structure of BaMo/sub 6/S/sub 8/ has been studied over the temperature range 19 K to 573 K by time-of-flight neutron powder diffraction. Below 175 K the data can be suitably refined in a triclinic, P1, cell with volume equal to the rhombohedral, R3, cell common to most Chevrel-phase structures. At temperatures immediately above 175 K, the rhombohedral, R3, Bragg peaks are broadened by satellite reflections which appear to be identical to those recently observed at low temperature in PbMo/sub 6/S/sub 8/ and SnMo/sub 6/S/sub 8/. An abrupt change in the sign of the temperature dependence of the hexagonal c axis (par. deltac/par. deltaT) signals the transition to an undistorted rhombohedral, R3, structure at temperatures above about 350 K. An extended Landau theory determines both continuous and discontinuous transitions from R3 induced by a single order parameter. Analysis of the order parameters inducing commensurate transitions imposes symmetry restrictions on the atomic displacements in the lower symmetry phases. The assumption of an R3 commensurate phase is not consistent with the bond lengths obtained for the distortions to the P1 (or P1) phase for any of the possible cells preserving order parameters. Thus the phase immediately above 175 K cannot be a commensurate R3 structure. This is consistent with experimental evidence. 25 refs., 11 figs., 8 tabs.
Date: January 1, 1986
Creator: Jorgensen, J.D.; Hinks, D.G.; Hatch, D.M. & Putnam, R.M.
Partner: UNT Libraries Government Documents Department

Charge delocalization and structural response in layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}: Enhanced Jahn-Teller distortion in the metallic regime

Description: Temperature dependent neutron diffraction studies on melt-grown crystals of the layered compound La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} reveal a dramatic structural response to the onset of ferromagnetism and the coincident insulator-metal transition. Unlike the related manganite perovskites, whose Jahn-Teller distorted octahedra become more regular at temperatures below the metal-insulator transition, the MnO{sub 6} octahedra of this layered material are more severely distorted when the charge is itinerant than when it is localized.
Date: November 1996
Creator: Mitchell, J. F.; Argyriou, D. N. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

On the pressure induced phase of Na{sub 2}CsC{sub 60}

Description: Neutron powder diffraction at pressures to 6 kbar in gaseous Ne has been used to study the pressure-induced phase transition and compressibilities of Na{sub 2}CsC{sub 60}. The pressure-induced phase can be achieved by compression to about 5 kbar at room temperature. If cooled, this phase can be retained below 200 K upon release of the pressure. The structure is orthorhombic as previously reported (but may differ in its detailed crystal structure) with lattice constants near 80 K and ambient pressure of a=9.385 A, b=10.06 A, and c=14.36 A. Corresponding linear compressibilities are 0.0004, 0014, and 0.0017 kbar{sup -1}, respectively. Identical pressure temperature cycling results in a superconductor with an unexpectedly low pressure dependence for {Tc} while in this phase. Models for the superconducting behavior of this compound are discussed.
Date: June 1996
Creator: Morosin, B.; Schirber, J. E.; Jorgensen, J. D.; Kwei, G. H.; Yildirim, T. & Fischer, J. E.
Partner: UNT Libraries Government Documents Department

Synthesis, structure, and superconductivity of La{sub 2-x}K{sub x}CuO{sub 4}

Description: Structural and superconducting properties of La{sub 2-x}K{sub x}CuO{sub 4} were studied by neutron and x-ray powder diffraction, ac magnetic susceptibility, and dc electrical resistivity. Superconductivity is observed in both orthorhombic and tetragonal phases with x{ge}0.06 and is absent for x{le}0.04. At room temperature, the system undergoes an orthorhombic-to-tetragonal phase transition at x{similar_to}0.07. Diffraction studies show no evidence of a low temperature tetragonal phase at 50 K. Comparison of these structural and superconducting properties with earlier data indicate that K contents have previously been highly overestimated.
Date: May 1, 1994
Creator: Keane, P.M.; Hinks, D.G. & Jorgensen, J.D.
Partner: UNT Libraries Government Documents Department

Increase of magnetic transition temperatures by reduction of local disorder for perovskite manganites.

Description: We report the synthesis of Sr{sub 1-x}Ca{sub x}MnO{sub 3} and La{sub 0.5}Ba{sub 0.5}MnO{sub 3} perovskites over extended cation and oxygen composition ranges and describe the dependence of their phase stability on the tolerance factor t = t(x,T,{delta}) that is a function of composition, temperature, and oxygen content. We show that magnetic transition temperatures depend strongly on the tolerance factor and charge disorder while dependence on the structural disorder is less important. By reducing charge and structural disorder we have significantly increased the Curie and Neel temperatures for perovskite manganites.
Date: July 2, 2002
Creator: Dabrowski, B.; Chmaissem, O.; Mais, J.; Kolesnik, S.; Jorgensen, J. D. & Short, S.
Partner: UNT Libraries Government Documents Department

Structural relationships in high temperature superconductors

Description: The recent discovery of two types of metallic copper oxide compounds which are superconducting to above 90/sup 0/K has renewed interest in the search for new high temperature superconducting materials. It is significant that both classes of compounds, La/sub 2-x/Sr/sub x/CuO/sub 4-y/ and YBa/sub 2/Cu/sub 3/O/sub 7-delta/ are intimately related to the extensively studied perovskite family. Both compounds contain highly oxidized, covalently bonded Cu-O sublattices, however, they differ in geometry. In this paper we discuss the relationship of these features to the superconducting properties. 30 refs., 6 figs.
Date: September 1, 1987
Creator: Schuller, I.K.; Segre, C.U.; Hinks, D.G.; Jorgensen, J.D.; Soderholm, L.; Beno, M. et al.
Partner: UNT Libraries Government Documents Department

Argonne pulsed neutron source program

Description: The development of the pulsed spallation neutron source and the associated experimental facility is described. The neutron scattering instrumentation includes several diffractometers and spectrometers.
Date: January 1, 1979
Creator: Carpenter, J.M.; Crawford, R.K.; Peterson, S.W.; Mueller, M.H.; Jorgensen, J.D. & Reis, A.H. Jr.
Partner: UNT Libraries Government Documents Department

Argonne pulsed neutron source program

Description: The ZING-P' prototype pulsed spallation neutron source based on the 500 MeV Booster II proton synchrotron at Argonne is currently operating at 15 hz and 8 x 10/sup 12/ neutrons/pulse. Four neutron scattering instruments, two elastic and two inelastic, have been erected. In operation are 1% and 0.3% ..delta..d/d resolution powder diffractometers, a crystal analyzer spectrometer sensitive to excitations with E < 0.3 eV and a chopper spectrometer sensitive to E < 1 eV. Pulsed beam single crystal and a small-angle scattering instrument and a neutron bottle experiment are in construction stage. The peak thermal neutron flux at the beam sources is 10/sup 14/n/cm/sup 2/-sec. IPNS-I, an advanced pulsed source also based on Booster II, is scheduled for 1979 construction and is to begin operation in 1981. Utilizing a U/sup 238/ target it will produce 10/sup 14/ neutrons / pulse, operate at 45 hz and yield a peak thermal flux of 8 x 10/sup 14/ n/cm/sup 2/-sec and an epithermal flux of 2 x 10/sup 15/ n/cm/sup 2/-sec eV at 1 eV. Twelve advanced time-of-flight neutron scattering instruments are being designed for installation at IPNS-I. Accelerator design studies and other planning for IPNS-II, a 10/sup 16/ flux source, are continuing. The design of the single crystal time-of-flight neutron diffractometer is based on the Laue technique utilizing a broad thermal neutron spectrum (0.75 to 5.0 A). It will employ a /sup 3/He filled, multiwire, position-sensitive, area detector of the Borkowski-Kopp type. The detector will be interfaced to a large computer through a 1 megabyte fast storage buffer memory. Data collection will involve simultaneous measurement of 10/sup 2/ - 10/sup 3/ reflections with stationary crystal and detector.
Date: January 1, 1978
Creator: Carpenter, J.M.; Crawford, R.K.; Peterson, S.W.; Mueller, M.H.; Jorgensen, J.D. & Reis, A.H. Jr.
Partner: UNT Libraries Government Documents Department

Structural features that optimize high temperature superconductivity

Description: For example, various defects can be introduced into the blocking layer to provide the optimum carrier concentration, but defects that form in or adjacent to the CuO{sub 2} layers will lower T{sub c} and eventually destroy superconductivity. After these requirements are satisfied, the highest T{sub c}`s are observed for compounds (such as the HgBa{sub 2}Ca{sub n-1}CuO{sub 2n{plus}2{plus}x} family) that have flat and square CuO{sub 2} planes and long apical Cu-O bonds. This conclusion is confirmed by the study of materials in which the flatness of the CuO{sub 2} plane can be varied in a systematic way. In more recent work, attention has focused on how the structure can be modified, for example, by chemical substitution, to improve flux pinning properties. Two strategies are being investigated: (1) Increasing the coupling of pancake vortices to form vortex-lines by shortening or ``metallizing`` the blocking layer; and (2) the formation of defects that pin flux.
Date: January 1, 1996
Creator: Jorgensen, J.D.; Hinks, D.G. Chmaissem, O.; Argyriou, D.N.; Mitchell, J.F. & Dabrowski, B.
Partner: UNT Libraries Government Documents Department