D. R. Richards

Argonne National Laboratory, Lemont, Illinois, United States

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Publications (71)217.13 Total impact

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    Full-text · Article · Jan 2011 · MRS Online Proceeding Library

  • No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • Y. Shen · D.R. Richards · D.G. Hinks · A.W. Mitchell
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    ABSTRACT: A series of samples along the composition lines Bi{sub 2+{ital x}}Sr{sub 2{minus}{ital x}}CuO{sub {ital y}} and Bi{sub 2}Sr{sub 2{minus}{ital x}}CuO{sub {ital y}} have been used to study the structural modulation, chemistry, and superconducting properties of pseudo-tetragonal Bi{sub 2}Sr{sub 2}CuO{sub 6}(2201). The 2201 phase can be formed from crystallization of thin glassy platelets. The sample displayed a strong crystallographic (00{ital l}) orientation which made it possible to determine incommensurate modulations near (00{ital l}) reflections using a conventional x-ray {theta}-2{theta} scan. From the crystallization of the 2201 phase, it was found that structural modulation was intrinsic to the phase, and ordering of the structure required a long time at high temperature. High temperature {ital in} {ital situ} x-ray diffraction of a 2201 Bi{sub 2}Sr{sub 1.85}CuO{sub {ital y}} platelet showed that the modulation existed at 875 {degree}C in O{sub 2} ({ital T}{sub melt} {approx}892 {degree}C in O{sub 2}). These suggest that the structural modulation cannot be caused solely by oxygen ordering and that metal-ion displacement must be involved. By removing 0.04 to 0.05 oxygen atom per formula unit from Bi{sub 2}Sr{sub 2}CuO{sub {ital y}} and Bi{sub 2}Sr{sub 1.85}CuO{sub {ital y}}, the {bold c}* components of the modulation changed from 0.31 to 0.26 and from 0.38 to 0.31, respectively, while the {bold b}* component of the modulation remained approximately 0.2. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation. However, the invariance of {ital b}{sub mod} with respect to the change in oxygen content does not support the model that explained the modulation by inserting extra oxygen in the BiO plane.
    No preview · Article · Mar 1992 · Journal of Materials Research
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    ABSTRACT: The phonon densities of states (DOS) of insulating BaBiO3 and superconducting Ba0.6K0.4BiO3 and the variation of the phonon spectrum of the superconducting compound upon oxygen-isotope (16O, 18O) substitution are determined by inelastic neutron scattering (INS) and molecular-dynamics (MD) simulations. The MD simulations are carried out with an effective interaction potential which includes steric effects, Coulomb interactions, and the charge-dipole interactions due to the electronic polarizability of O2-. The MD results are in good agreement with the INS experiments and electron-tunneling measurements. Partial DOS of Ba, K, Bi, and O in BaBiO3 and Ba0.6K0.4BiO3 are also determined from MD simulations. In the superconducting material, the phonon spectrum softens and is comprised of broad bands around 15, 30, and 60 meV. The partial DOS reveal that phonons above 20 meV are due to oxygen vibrations, whereas phonons below 20 meV are due to Ba, K, and Bi. The reference oxygen-isotope-effect exponent, αOr=-∂ ln〈ω〉/∂ lnMO, of Ba0.6K0.4BiO3 is determined to be αOr=0.42±0.05 from the mass (MO) variation of the first moment of the phonon DOS, O16〉 and O18〉.This value is close to the oxygen-isotope-effect exponent, αO, determined from the variation of Tc (0.41±0.03 by Hinks et al. and 0.35±0.05 by Kondoh et al.), indicating that Ba0.6K0.4BiO3 is a weak- to moderate-coupling BCS-like superconductor and that the high Tc (∼30 K) results from large electron-phonon matrix elements involving high-energy oxygen-related phonons.
    No preview · Article · Mar 1992 · Physical review. B, Condensed matter
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    ABSTRACT: Single phase samples of (Ba1-xKx)(Bi1-ySby)O3 have been synthesized over a limited compositional range using a two-step procedure: high-temperature firing in a low oxygen partial pressure followed by low-temperature oxygen annealing. For y > 0 samples are superconducting only in a simple cubic structure with Tc gradually decreasing as y increases for fixed x. As in the case of potassium only (y=0) substituted material, the highest Tc is found in the compositional range adjacent to the structural phase transition into the lower symmetry nonsuperconducting phase. Mössbauer effect measurements indicate that Sb is close to +5 valent state with small mixed valent behavior which increases with Sb content.
    No preview · Article · Feb 1992 · Physica C Superconductivity
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    ABSTRACT: The incommensurate modulation in Ba1-xKxBiO3 is studied by means of electron diffraction. Attention is paid to the symmetry of the modulation and the effect of electron irradiation. Interpretation of the modulation symmetry in cubic samples (0.3 < x < 0.5) as well as in orthorhombic samples (0.1 < x < 0.3) in the framework of a four-dimensional description allows one to determine it as a periodic displacement of the bismuth atoms. Satellite reflections corresponding to the modulation are observed only after irradiation with electrons, However, since the critical voltage is below 40 kV, it is believed that the modulation is in fact a fundamental material property. The incommensurability of the modulation for x < 0.4 results in a partial charge disproportionation on the bismuth sites, which can explain the lack of superconducting properties for Ba1-xKxBiO3 with x < 0.4. In the superconducting phase Ba0.6K0.4BiO3, the modulation is commensurate, and high-resolution observations provide evidence for a modulation model without charge disproportionation, yielding a metallic behavior at room temperature.
    No preview · Article · Dec 1991 · Physical review. B, Condensed matter
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    ABSTRACT: A combination of neutron powder diffraction and thermogravimetric analysis has been used to study the structural phase behavior of Ba1-xKxBiOy under conditions that simulate synthesis. On heating in 1% O2, the cubic perovskite Ba0.6K0.4BiO3 decomposes to a mixture of Ba1-xKxBiOy with x < 0.4 and KBiO2. Further increase in temperature causes the reincorporation of potassium into the perovskite at the expense of KBiO2. At 720°C, the sample is again a single-phase cubic perovskite with x = 0.4 containing a large concentration of oxygen vacancies. If this sample is then cooled in argon, no chemical phase decomposition occurs. Instead, the cubic oxygen-deficient perovskite transforms to an orthorhombic oxygen-vacancy-ordered phase. A subsequent low-temperature anneal in pure oxygen fills the oxygen vacancies while retaining the potassium in the lattice, resulting in a transformation back to the cubic perovskite. This work shows that fully oxygenated Ba1-xKxBiO3 for x > 0.1, including the superconducting compositions, is metastable, and supports the hypothesis that the creation of oxygen vacancies is necessary to allow the substitution of K1+ for Ba2+ in the lattice.
    No preview · Article · Nov 1991 · Journal of Solid State Chemistry
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    ABSTRACT: A combination of neutron powder diffraction and thermogravimetric analysis has been used to study the structural phase behavior of Ba{sub 1-x}KâBiO{sub y} under conditions that simulate synthesis. On heating in 1% Oâ, the cubic perovskite Ba{sub 0.6}K{sub 0.4}BiOâ decomposes to a mixture of Ba{sub 1-x}KâBiO{sub y} with x < 0.4 and KBiOâ. Further increase in temperature causes the reincorporation of potassium into the perovskite at the expense of KBiOâ. At 720 C, the sample is again a single-phase cubic perovskite with x = 0.4 containing a large concentration of oxygen vacancies. If this sample is then cooled in argon, no chemical phase decomposition occurs. Instead, the cubic oxygen-deficient perovskite transforms to an orthorhombic oxygen-vacancy-ordered phase. A subsequent low-temperature anneal in pure oxygen fills the oxygen vacancies while retaining the potassium in the lattice, resulting in a transformation back to the cubic perovskite. This work shows that fully oxygenated Ba{sub 1-x}KâBiOâ for x > 0.1, including the superconducting compositions, is metastable, and supports the hypothesis that the creation of oxygen vacancies is necessary to allow the substitution of K{sup 1+} for Ba{sup 2+} in the lattice.
    No preview · Article · Nov 1991 · Journal of Solid State Chemistry
  • Y. Shen · D. R. Richards · D. G. Hinks · A. W. Mitchell
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    ABSTRACT: High‐temperature x‐ray diffraction has been carried out on a superconducting single‐phase sample of pseudo‐tetragonal Bi 2 Sr 2 CuO 6 + y , which was obtained from crystallization of thin glassy platelets. The sample displayed (00l) preferred orientation, which made it possible to determine the incommensurate modulation by conventional θ-2θ x‐ray scans. The existence of the modulation peaks (800 °C) indicates that the modulation cannot be caused solely by oxygen ordering. As temperature increased, it was observed that a decrease in the intensity of modulation lines was associated with an increase in intensity of fundamental reflection, which was similar to the order‐disorder phase transition. By removing 0.045(5) oxygen atom per formula unit, the components of modulation satellites along the b* and c* directions changed from b mod =0.183(9) and c mod =0.315(4) to 0.184(6) and 0.260(4), respectively. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation. However, the invariance of b mod with respect to the change in oxygen content does not support the model which explained the modulation by inserting extra oxygen in BiO plane.
    No preview · Article · Oct 1991 · Applied Physics Letters
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    ABSTRACT: A pair distribution function (PDF) analysis of pulsed neutron powder diffraction data has been carried out to investigate the local atomic structure of Nd2-xCexCuO4-y, x=0.165, over a range of temperatures close to Tc (Tc=22 K). Simulation of the PDFs using structural models indicate that, at all temperatures up to 50 K, the local atomic structure is significantly distorted from the T' structure. Both 0(1) (in-plane) and 0(2) (out-of-plane) atoms are displaced from their average sites. Furthermore, we see evidence of a structural modification occurring between 25 and 40 K. Such a change is not observed in a nonsuperconducting x=0.2 sample down to 10 K, which suggests that this modification may be connected with the superconductivity.
    No preview · Article · Sep 1991 · Physica C Superconductivity
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    ABSTRACT: An irreversibility line has been identified in bulk Ba0.625K0.375BiO3, which follows the relation 1-T/Tc~H2/3. According to the flux-creep model, the magnetic irreversibility is caused by thermally activated flux creep and is closely connected with the field-induced resistive broadening. However, previous resistivity measurements showed that the Ba0.625K0.375BiO3 system did not exhibit resistive transition broadening. Thus, an alternative explanation based on the Josephson-coupling model is suggested to interpret the irreversibility observed in Ba0.625K0.375BiO3.
    No preview · Article · Aug 1991 · Physical review. B, Condensed matter
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    ABSTRACT: The variation of the phonon spectrum of Ba0.6K0.4BiO3 upon oxygen isotope substitution is determined by inelastic neutron scattering (INS). By combining the INS results with molecular-dynamics computer simulation, a value of 0.42±0.05 is obtained for the reference isotope-effect exponent. This is close to the isotope-effect exponent determined from the variation of Tc upon oxygen substitution, indicating that Ba0.6K0.4BiO3 is a weak-coupling superconductor with large matrix elements coupling the electrons to high-energy phonons involving oxygen vibrations.
    No preview · Article · Jul 1991 · Physical Review Letters
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    ABSTRACT: A new Bi(III) oxide, BaBiO2.5, has been prepared by slow-cooling of the perovskite-like BaBiO3 in a reducing atmosphere. The structure was solved ab initio from a combination of synchrotron X-ray and pulsed neutron powder diffraction data. The phase crystallizes in the monoclinic space group P21/c, with a = 7.3412(1) Å, b = 7.5793(1) Å, c = 6.0722(1) Å, and β = 99.187(2)°. A novel layer-type structure is adopted consisting of double BiOx sheets in the bc plane separated by Ba ions along the a direction. Bi is four-coordinated in a pseudotrigonal bipyramidal environment, with the electron lone-pair occupying one of the equatorial sites, similar to that found in β-Bi2O3. There are, thus, significant differences from either the Aurivillius phases or the high-Tc bismuth cuprate superconductors, which may also be regarded as layered bismuth oxides.
    No preview · Article · Jun 1991 · Journal of Solid State Chemistry
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    ABSTRACT: The local structure around Bi atoms has been studied in Ba1-xKxBiO3 (0.0<x<0.4) via x-ray-absorption spectroscopy. The structure is determined in terms of coordination numbers and interatomic distances around the Bi atoms between 300 and 13 K from Bi L3 extended x-ray-absorption fine-structure data. The charges on Bi and O have been measured as a function of K doping using Bi L3, and O K-edge x-ray-absorption near-edge-structure spectroscopy. It was found that the ordered breathing-mode-type distortion in BaBiO3 is replaced by a disordered distortion of the same type when K is substituted into the Ba site, which persists down to 13 K for x=0.2 and disappears for the x=0.4 composition. The overall Bi valence increases only slightly whereas O 2p orbitals are doped with holes with increasing K content. As the insulator-metal boundary is crossed for x=0.35 these holes become delocalized, and contribute to the metallic behavior of the Ba0.6K0.4BiO3 compound.
    No preview · Article · Apr 1991 · Physical review. B, Condensed matter
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    ABSTRACT: The chemical behavior of Ba1 − xKxBiO3 − δ at high temperature is investigated. The solubility of potassium in BaBiO3 is a strong function of temperature: as the temperature is increased for superconducting Ba0.6K0.4BiO3 (at constant PO2), potassium is initially lost from the lattice and is then reincorporated at higher temperatures. The superconducting, oxygen-stoichiometric compositions are probably metastable under ambient conditions. At room temperature, the oxygen-stoichiometric material undergoes a series of crystallographic phase transitions as the potassium content is increased. These transitions may be thought of as the logical removal of both the symmetric breathing mode and the rigid tilt distortions of the BiO6 octahedra. Eventually at x = 0.37, all static distortions are removed and the material becomes cubic and superconducting.
    No preview · Article · Feb 1991 · Journal of the Less Common Metals
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    ABSTRACT: An irreversibility line has been identified in bulk Ba0.625K0.375BiO3, which follows the relation 1-T/Tc∼H2/3. According to the flux-creep model, the magnetic irreversibility is caused by thermally activated flux creep and is closely connected with the field-induced resistive broadening. However, previous resistivity measurements showed that the Ba0.625K0.375BiO3 system did not exhibit resistive transition broadening. Thus, an alternative explanation based on the Josephson-coupling model is suggested to interpret the irreversibility observed in Ba0.625K0.375BiO3.
    No preview · Article · Jan 1991 · Physical review. B, Condensed matter
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    ABSTRACT: Single-phase samples of Ba(1-x)K(x)BiO3 (where x is between 0 and 0.5) are synthesized from the melt followed by a two-step annealing procedure. Potassium is incorporated into the oxygen-deficient material under reducing conditions. Low-temperature oxygen annealing fills the oxygen vacancies without potassium loss. Samples are bulk superconductors with a simple cubic structure for x of more than 0.35 with Tc gradually increasing as x increases. At 10 K, with decreasing potassium content, the symmetry reduces to tetragonal at x = 0.35, orthorhombic at 0.2 and finally to monoclinic at 0.1. Normal state resistivity displays metallic behavior for values between 0.40 and 0.5, phonon-assisted variable-range hopping behavior for values between about 0.15 and 0.30, and semiconducting behavior for values of x less than 0.15.
    No preview · Article · Jan 1991
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    ABSTRACT: Observation of similar behavior in three different superconducting oxides suggests that the maximum Tc is limited by phase instability. These conclusions are based on previously published neutron powder diffraction data for La2CuO4+x and Nd2−XCexCuO4 and new data for La2−xSrxCuO4 which show a phase separated region for compositions adjacent to that which produces the highest Tc.
    No preview · Chapter · Jan 1991
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    ABSTRACT: A series of Nd2-xCexCuO4 samples with starting compositions 0≤x≤$0.25 have been prepared in several different oxygen partial pressures at 990-1100°C. The high resolution neutron and synchrotron X-ray diffraction data show that single-phase material is formed only for the x=0.0 and x approximately 0.165 compositions. For the x = 0.165 sample there is a strong dependence of cerium solubility on synthesis temperature.
    No preview · Article · Jan 1991
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    ABSTRACT: Using in situ neutron powder diffraction, we have studied the structures of BaBiO3−δ at high-temperature. Above 520°C, fully oxygenated BaBiO3 has a double cubic perovskite structure (space group Fm3̄m; ) exhibiting a commensurate charge density wave (CDW). Depleting oxygen destroys this commensurate CDW, giving rise to a primitive cubic perovskite structure with one Bi site (space group Pm3̄m; ). Further reduction in oxygen content leads to a pseudo-tetragonal phase with δ≈0.5. We propose that oxygen deficient BaBiO3−δ in the primitive cubic phase could be metallic as a result of losing the commensurate CDW.
    No preview · Article · Dec 1990 · Materials Research Bulletin