E.M. McCarron

The incommensurate structure of (Sr14−xCax)Cu24O41 (0 < x ∼ 8) a superconductor byproduct

Abstract The structures of two compositions of the title compound, Sr 8 Ca 6 Cu 24 O 41 and Sr 14 Cu 24 O 41 , have been determined by x-ray, single-crystal diffraction techniques. The structure of the first is best described as two interpenetrating structures: one with (Sr,Ca)(Cu 2 O 3 sheets)(Sr,Ca) layers in an orthorhombic cell with dimensions of a = 11.375(2), b = 12.9027(2), c = 3.910(1) A and a second structure with layers of (CuO 2 chains) in a cell having identical a and b values but with c = 2.753 A . The title formulation arises because these two subcells are nearly commensurate at 7 × c (sheet) = 27.372(2) and 10 × c (chain) = 27.534. The structure of the pure strontium phase is very similar but the adjacent CuO 2 chains in a given layer shift to a less constrained conformation: a = 11.459(2), b = 13.368(5), c (sheet) = 3.931(1), c ( chain ) = 2.749(1) A . X-ray powder diffraction studies have shown that x may vary from 0 to ∼ 8, with the layer axis, b, showing the most sensitivity to composition. Single crystals of these phases were found to be semiconductors.

Influence of oxygen stoichiometry on the structure and superconducting transition temperature of YBa2Cu3Ox

A detailed study of the superconducting properties and the crystal symmetry of YBa2Cu3Ox as a function of oxygen content (x) is presented. We correlate the oxygen content, structure and superconducting transition temperature for YBa2Cu3Ox (6<×<7) series of materials formed by the topotactic intercalation/deintercalation of oxygen. It is shown that the orthorhombic to tetragonal phase transition coincides with a loss in superconductivity for samples prepared both by quenching from high temperature and samples prepared by deoxygenation at low temperature. For the orthorhombic phase, Tc monotonically decreases as x goes from 7.0 to 6.4 along with a complementary decrease in the extent of orthorhombic distortion. The decrease in Tc, however, is not uniform. For quenched samples it shows a plateau for x ∼ 6.75 to 6.55 and then a rather abrupt drop around x ∼ 6.5. Comparison of our data with the literature indicates that the dependence of superconducting properties and crystal structure on the oxygen content can be a complex function of sample processing history. Samples with the same oxygen content but prepared in different ways may have x-ray powder patterns that are indistinguishable, but significantly different electrical properties.

Observation of superconductivity in (Y1−xCax)Ba2Cu3O6

Abstract Substitution of Ca 2+ for Y 3+ in YBa 2 Cu 3 O 6 converts this tetragonal semiconductor into a tetragonal superconductor with a T c of 50 K. Furthermore, the structure of (Y 0.8 Ca 0.2 )Ba 2 Cu 3 O 6 , determined by neutron powder diffraction, is fully consistent with partial oxidation of the Cu(2) O sheets, whereas the linear O Cu(1) O units do not undergo oxidation. Observation of a limited solid solubility for Ca 2+ in this structure is attributed to a maximum allowed oxidation state for Cu(2). This implies that in the superconductor YBa 2 Cu 3 O 7 the Cu O chains are responsible for the T c of 90 K, whereas the Cu O sheets alone produce superconductivity at 50 K in YBa 2 Cu 3 O 6.5 .

The composition and proposed structure of the alkali metal layered molybdenum bronzes

Abstract The chemistry of the layered molybdenum bronzes with alkali metals as the guest cations was investigated. The reaction of MoO 3 and dithionete ion, S 2 O 4 2− , in neutral, buffered solutions resulted in the formation of bronzes, [A + (H 2 O) n ] x MoO 3 x− where A = Li, Na, K, Rb or Cs, and eliminated the problem of the co-intercalation of adventitious protons. Chemical analysis of these bronzes shoed x to be ∼ 0.25 and thermogravimetric analysis was used to determine the number of intercalated water molecules, n. The interlayer spacings of both air-dried ( AD ) and vacuum dried ( VD ) products were measured, and models to account for these spacings were proposed. Electron diffraction from the ( hOl ) planes of [Na(H 2 O) 2 ] 0.25 MoO 3 ( VD ) allowed the powder X-ray diffraction pattern to be indexed on an orthorhombic unit cell with a = 3.876(2), b = 19.093(7) and c = 3.733(3) A . The systematic absences were consistant with space group Ammm . The powder pattern of the more fully hydrated bronze [Na(H 2 O) 5 ] o.25 MoO 3 ( AD ) was also indexed on a related orthorhombic cell with a = 3.884(1), b = 22.618(8) and c = 3.751(2) A . The systematic absences in this case are compatable with Cmcm symmetry.

The growth and single crystal structure of a high pressure phase of molybdenum trioxide : MoO3-II

Abstract Single crystals of a new high pressure modification of molybdenum trioxide, MoO3-II, were grown in a tetrahedral anvil apparatus at elevated temperature. The structure of MoO3-II is monoclinic, P2 1 m , with unit cell parameters: a = 3.954(1) A, b = 3.687(2) A, c = 7.095(4) A, and β = 103.75(4)°. MoO3-II (4.75 g/cm3) is metastable at ambient pressure and converts to less dense orthorhombic α-MoO3 (4.71 g/cc) rapidly at temperatures above ∼200°C. Like the α-MoO3 structure, the structure of MoO3-II is layered. In fact, the individual MoO 3 3 O 2 2 O 1 1 layers of the two phases are virtually identical. However, the stacking sequence of the layers of MoO3-II (aaa) differs from that of α-MoO3 (aba). This is equated with an improved packing efficiency for the layers of MoO3-II versus those of α-MoO3.

β-MoO3 produced from a novel freeze drying route

Powdered samples of {beta}-MoO{sub 3} have been produced by the gentle heat treatment of freeze-dried molybdic acid at 350{degree}C for 1 hr. The samples, yellow-green in appearance, contained varying amounts of the thermodynamically stable {alpha}-MoO{sub 3}, depending upon the time and temperature of heat treatment. Neutron diffraction data were collected at 300 K. all peaks, not attributable to {alpha}-MoO{sub 3}, were indexed on the basis of a monoclinic cell, P2{sub 1}/c, {alpha} = 7.1228(7), b = 5.3660(6), c = 5.5665(6), {beta} = 92.01(1){degree}, V = 212.62(6){angstrom}{sup 3}. The structure, which is related to ReO{sub 3}, contains two crystallographically independent octahedra. Both show evidence of disorder at the Mo and O sites. Two distinct orientations of a short mo-O distance, suggestive of the type of molybdenyl bond observed in both the {alpha} and {beta}{prime}-forms, are primarily responsible for the observed disordering.

Transient photoinduced conductivity in semiconducting single crystals of YBa2Cu3O6.3: Search for photoinduced metallic state and for photoinduced superconductivity

Abstract We present the results of time resolved experiments on single crystals of YBa 2 Cu 3 O 6.3 mounted in the Auston switch configuration. At 300K, the peak photocurrent is linear in the pump light intensity at low light levels and saturates at the highest light levels available. Below 200K, the peak photocurrent increases approximately linearly with the pump light intensity at low light levels, but shows an increase to superlinear behavior above ∼5x10 15 photons/cm 2 , indicative of the onset of an additional transport mechanism. At 50K, the transient decrease in the resistance indicates a photoinduced change in resistivity within the absorption depth of the light by more than 14 orders of magnitude. The photoinduced conductivity is time delayed with respect to the absorption; approximately 0.5 ns is required for the photoexcited system to evolve to a highly conducting state. The results are discussed in terms of the possibility of a photoinduced transition to the metallic state and of photoinduced superconductivity.

The structure of the 44 K superconductor (Y1−xCax)Ba2Cu3O6+δ (δ ≤ 0.2)

Abstract Replacing Y( III ) by Ca( II ) in (Y 1− x Ca x )Ba 2 Cu 3 O 6 induces superconductivity. A solid-solution limit exists, as is evidenced by the lack of variation in unit cell parameters for values of x > 0.3. At low nominal doping levels ( x 2 Cu 3 O 6 and BaCuO 2 . At higher nominal doping levels the Ca content of the 123-type phase becomes constant ( x ∼ 0.30). Structural refinements using data collected by X-ray and neutron powder diffractometry are consistent with Ca-doping causing oxidation only of Cu in the sheet, leaving the copper in the so-called chain site as Cu(I). Many of the structural changes occurring in this system upon oxidation, such as a flattening of the CuO 2 sheets, a contraction of the in-plane CuO distance, and a decrease in the CuCu distance between planes, also occur in the structurally related superconducting system, Pb 2 Sr 2 (Y 1− x Ca x )Cu 3 O 8 .

Magnetic-field dependence of the low-temperature specific heat of some high-Tc copper-oxide superconductors evidence for an H12T contribution in the mixed state

Abstract Measurements of the magnetic-field dependence of the low-temperature specific heat of YBa 2 Cu 3 O 7 and Ca and Sr doped La 2 CuO 4 are reviewed, with an emphasis on recent measurements on a YBa 2 Cu 3 O 7 sample in a number of applied magnetic fields ( H ) between 0 and 7 T at temperatures ( T ) between ∼ 0.4 and 10 K. The major field-dependent components of the specific heat of that sample are a T proportional term an a contribution attributed to Cu 2+ magnetic moments. In contrast with most earlier results, or their interpretation, the T proportional term is not linear in H . Within the experimental uncertainty it agrees with the H 1 2 T dependence predicted by Volovik for a superconductor with lines of nodes in the energy gap. In that respect, the results are similar to those obtained in a recent Stanford/University of British Columbia collaboration. However, there is no convincing evidence for the presence of the T 2 term in the zero-field specific heat that is also expected for lines of nodes in the gap. Possible reasons for this apparent discrepancy are discussed. Less detailed data on other samples are consistent with these findings. For H = 0, the contribution associated with the Cu 2+ moments, the “zero-field upturn”, is the high-temperature tail of a broad anomally produced by internal interactions. With increasing H it evolves into a form more closely approximated by simple Schottky functions. The H dependence of this contribution to the specific heat is consistent with that expected for a low concentration of rmspin- 1 2 moments with a distribution of internal fields that is progressively narrowed with increasing H . It is not consistent with the H proportional prefactor associated with the Schottky anomalies that has been reported elsewhere.

Topochemical reduction of the high temperature superconductor YBa2Cu3O7: The structure of YBa2Cu3O6 by neutron diffraction

Abstract Superconducting orthorhombic YBa 2 Cu 3 O 6.8 has been reduced to tetragonal YBa 2 Cu 3 O 6 without structural collapse by heating under a nitrogen atmosphere at 900°C. Further reduction above ∼950°C results in destruction of the perovskite-related structure. Neutron powder diffraction data were fitted by the Rietveld profile refinement method to obtain the structure of YBa 2 Cu 3 O 6 . The reduced material is viewed as containing isolated linear Cu 1+ O 2 units and square pyramidal Cu 2+ O 5 units that share oxygen atoms to form a two-dimensional copper-oxygen network. Semiconducting behavior is observed for YBa 2 Cu 3 O 6 from 4.2 K to 300 K consistent with such a localized-electron model. The structural refinement of YBa 2 Cu 3 O 6 shows that the linear (Cu − 0) n chains present in YBa 2 Cu 3 O 7 are now completely absent.