B. Morosin

The crystal structure of superconducting La2CuO4.032 by neutron diffraction

Abstract The crystal structure of superconducting (37.5 K) La2CuO4.032 has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4 y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO6octahedra). The O4 insertion causes a 0.75 A displacement of 4.8% of the O1 atoms towards new O3 positions (x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3–O4 distance of 1.64(3) A. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8−, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La2CuO4.032, with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band.

Stoichiometry of bulk superconducting La2CuO4+δ: A superconducting superoxide?

Abstract A variety of analytical tools have been combined to show that bulk superconducting La2CuO4+δ has a La to Cu ratio of 2.00 (±0.02) and an oxygen stoichiometry of 4.13 (±0.1). The combination of these analytical results and magnetization data with a model leads us to suggest that the excess charged oxygen is introduced as O-2.

Intercalation of molecular species into the interstitial sites of fullerene

Molecular species were found to diffuse readily into the octahedral interstitial sites of the fcc lattice of C{sub 60}. The {sup 13}C NMR spectrum of C{sub 60} under magic angle spinning (MAS) conditions consisted of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The downfield shift obeys Curies law and is attributed to the Fermi-contact interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C{sub 60} molecules. Exposure of C{sub 60} to 1 kbar oxygen for 1.75 h at room temperature resulted in a spectrum of seven evenly spaced resonances corresponding to the filling of 0 to 6 of the adjacent octahedral interstitial sites with oxygen molecules. The distribution of site occupancies about a C{sub 60} molecule provided evidence that the intercalation process is controlled by diffusion kinetics. Exposure to 0.14 kbar hydrogen gas at room temperature for 16 h filled a substantial fraction of the interstitial sites of C{sub 60} and C{sub 70} with hydrogen molecules.

Structural and compositional characterization of polycrystals and single crystals in the Bi- and Tl-superconductor systems: Crystal structure of TlCaBa2Cu2O7

Abstract The growth of mm-sized crystals in the Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductor systems shows the common presence of polycrystals, which can be described as syntactic intergrowth crystals, as well as normal single crystals. Three distinct phases with superconducting transitions at 103 K, 106–108 K and 112–114 K have been grown in the Tl-Ca-Ba-Ca-O system. The newly identified 103 K phase is TlCaBa 2 Cu 2 O 7 which is structurally similar to YBa 2 Cu 3 O 7 but without the presence of chains. Transmission electron diffraction and electron microprobe analysis on several polycrystals show that they are composed of interlayers of these distinct phases grown epitaxially along the c -axis. Crystals have been grown for two phases in the Bi-Ca-Sr-Cu-O system that exhibit transition temperatures at 80 K and at 92 K. Oxygen annealing substantially improves the superconducting properties of most of the crystals.

Crystal Structures of Anhydrous Rare‐Earth Chlorides

The crystal structures at 298°K of LaCl3, NdCl3, EuCl3, and GdCl3 have been refined by full‐matrix least‐squares methods using three‐dimensional MoKα intensity data. These isomorphous compounds belong to the UCl3‐P63 / m structure type; metal atoms occupy symmetry sites at (13, 23, 14) and chloride ions lie on mirror sites at (x, y, 14) with x≅0.39 and y≅0.30. Standard deviations of the interatomic separations do not exceed 1 part in 1000.

Two-phase structural refinement of La2CuO4.032 at 15 K

Abstract A two-phase refinement of a crystal for the compound La2CuO4+δ (δ=0.032), based on the neutron diffraction data collected at 15 K with the D9 diffractometer at ILL, using ǂ=0.48 A , has been carried out. One phase (30%) consists of stoichiometric La2CuO4 domains, while the other (70%) of oxygen-rich La2CuO4.048 domains. The percentages of each phase, which have been refined together with the other parameters (scale factor, positional parameters, thermal factors and occupancy factors of the oxygen atoms), agree very well with the value determined from χAC measurements. The La2CuO4.048 structure is essentially the same as the average structure reported in ref. [1], the only difference being the oxygen content which is found to be δ= δ 0.70 . The extra oxygen, O(4), is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in the N2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the oxygen O(1) towards the O(3) positions. Different models are proposed for the distortion induced by this insertion according to the experimental value, 3.3 [6], found for the ratio of the amount of O(3) to that of O(4). If this ratio is assumed to be 3 and the O(3) atoms are localized about the insertion, then the formation of a short O(4)-O(3) bond would occur. The models not requiring the formation of the short bond correspond to ratios of 2 or 4. In the latter case there would be four displaced O(1), but due to the rigidity of the oxygen octahedra only two O(3) would be bonded to O(4), the other two being the apically opposite oxygen atoms of the same octahedra.

Sequential electron beam evaporated films of Tl2CaBa2Cu2Oy with zero resistance at 97 K

Thin unoriented polycrystalline films in the Tl‐Ca‐Ba‐Cu‐O system were prepared by sequential electron beam evaporation of multiple Tl, Ca, Ba, and Cu layers and a two‐stage anneal under controlled Tl and oxygen overpressures. These films show zero resistance as high as 97 K and transport critical current densities at 76 K up to 110 000 A/cm2 with typical values of 50 000 A/cm2. The critical currents exhibit a modest magnetic field dependence. Meissner effect data show an onset at 110 K and a superconducting fraction of 75%. Compositional and structural studies indicate that the films, although polyphase, are predominantly Tl2Ca1Ba2Cu2Oy. There is no evidence of other superconducting phases.

Crystal Structures of Manganese(II) and Iron(II) Chloride Dihydrate

The crystal structures at 298°K of MnCl2·2H2O and FeCl2·2H2O have been refined by means of three‐dimensional differential syntheses using diffractometer single‐crystal x‐ray‐intensity data. The crystal structures of these isomorphic compounds consist of polymeric chains of metal and chlorine ions arranged in a near‐square planar configuration with each chlorine ion shared by two metal ions. The water molecules which fill the remaining octahedral positions about the metal ion link adjacent chains, paralleling the c axis, by hydrogen bonds.

X‐Ray Diffraction and Nuclear Quadrupole Resonance Studies of Chromium Trichloride

The infinite‐layer compound CrCl3 is found to undergo a first‐order phase transformation near 240°K. The crystal structures of the high‐ and low‐temperature phases have been elucidated by detailed x‐ray diffraction and 35Cl nuclear quadrupole resonance studies. The previously reported structure for CrCl3 is shown to be incorrect. The present single‐crystal diffraction results at 298° and 225°K give monoclinic (C2/m: a0=5.959 A, b0=10.321, c0=6.114 A, β=108.49°) and rhombohedral (R3: a0=5.942 A, c0=17.333 A) structures, respectively. The 35Cl quadrupole resonance frequencies, measured in polycrystalline samples, have been compared with results of field‐gradient calculations based on the ionic point‐charge model. The ionic contribution accounts for a large fraction of the total gradient making it impossible to evaluate the covalency parameters of the Cr–Cl bonds from the measured quadrupole coupling constants.

Morphology control and high critical currents in superconducting thin films in the Tl-Ca-Ba-Cu-O system

Abstract Superconducting polycrystalline thin films in the Tl-Ca-Ba-Cu-O system have been prepared by electron beam evaporation followed by appropriate sintering and annealing. The configuration employed (free or confined surface) to sinter the films determines the morphology varying from random to highly oriented, respectively. These films are predominantly the Tl2Ca2Ba2Cu3O10 phase, but some contain up to 50 at % Tl2CaBa2Cu2O8, demonstrating a toleration of variable stoichiometry. The key to obtaining high quality materials with short air sintering times is precise control of the Tl and oxygen partial pressures. Transition temperatures to 110 K and transport critical current densities to 600 000 A/cm2 at 76 K have been attained. The best films (room temperature resistance p 1 mΩcm show a definite correlation between critical current and p, consistent with the presence of weak links.