M. François

A study of the Cu-O chains in the high Tc superconductor YBa2Cu3O7 by high resolution neutron powder diffraction

Abstract The structure of orthorhombic YBa2Cu3O7 was refined in the temperature interval 5–320K, to an accuracy higher by a factor of about two compared to previous refinements. No major structural change is observed at the onset of superconductivity but the lattice dimensions and some of the structural parameters show small anomalies near 90K and 240K. All vibrational amplitudes are normal except for those of the 04 atoms across the Cu-O chain, which are large. Good agreement with the data is obtained by assuming that the 04 atoms are located on potential minima away by at least 0.08A from the chain axis at all temperatures. Annealing near 240K to look for a possible order-disorder transition revealed no structural changes.

Crystal structures of high-T c oxides

A comprehensive review of structure work on high-Tc oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba1−x,Kx)BiO3 (Tc=30 K),II. (La2−x, Srx)CuO4 (Tc=40 K),III. (Nd, Ce, Sr)2CuO4 (Tc=28 K),IV. (Nd2−x, Cex)CuO4 (Tc=24 K),V. YBa2Cu3O7 (Tc=90 K),VI. YBa2Cu4O8 (Tc=80 K),VII. Y2Ba4Cu7O14 (Tc=40 K),VIII. Pb2Sr2NdCu3O8 (Tc=70 K),IX. TlBa2CaCu2O7 (Tc=103 K),X. TlBa2Ca2Cu3O9 (Tc=120 K),XI. Tl2Ba2CuO6 (Tc=90 K),XII. Tl2Ba2CaCu2O8 (Tc=112 K),XIII. Tl2Ba2Ca2Cu3O10 (Tc=125 K). Except forI (perovskite type),II (K2NiF4 type) andIV (Nd2CuO4 type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.

Raman spectra of single crystal CuO

Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm−1 (Ag, 344cm−1 (Bg) and 629cm−1 (Bg). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm−1. The temperature dependence of the linewidth of the Ag mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.

Structural phase transition at 150K in the high-temperature superconductor La1.85Sr0.15CuO4

Abstract Neutron powder diffraction analysis shows an orthorhombic deformation of the tetragonal K2NiF4 type structure below 150K. The structure at 7K was refined by the Rietveld method (space group C m c a, a=5.324(2) A , b=13.200(A) A , c=5.348(2) A , Rwp=0.07, 19 parameters). The phase transition leads to a tilting of the Cu centred 0 octahedra about one of the pseudotetragonal a axes by about 3°, similar to that in La2CuO4. The data set an upper limit of about 5% to the oxygen defect concentration.

Polarized light and X-ray precession study of the ferroelastic domains of YBa2Cu3O7-δ

The ferroelastic domains of the orthorhombic phase of YBa2Cu3O7-σ have been observed in polarized light on ceramics and single crystals. By combining polarized light microscopy with the X-ray precession technique, the correlation of the orientation of the orthorhombica- andb-axes with that of thea-b-plane bireflectance, reflection dichroism, transmission dichroism (at a thickness of about 1μm), reflection tints generated with compensators and upon uncrossing of polars, as well as the orientation of etch pits has been realized on ferroelastic single domains, bi-domains and more complicated domain patterns. Four ferroelastic orthorhombic domain states have been identified, at variance with former group theoretical considerations, predicting only two states. Ensembles of lamellar domains beyond optical resolution generate strong bireflectance with principal axes rotated by 45° relative to the truea, b-directions.

Structure of the high-temperature superconductor Ba2YCu3O7 by X-ray and neutron powder diffraction

Abstract The average structure at 8 K of Ba2YCu3O7 (critical temperature Tc = 90 K) was refined by the Rietveld method (space group Pmmm, a =3.817(2) A , b =3.883(2) A , c =11.633(7) A , Rwp = 0.11, 28 parameters, T = 8 K). It contains two Cu sites of which one has square-pyramidal ([ CuO ] = 2 × 1.925(1) A , 2 × 1.959(1) A (base), 2.285(6)A (apex) and the other square-planar ([CuO] = 2 × 1.850(5) A , 2 × 1.942(1) A ) oxygen coordination. The CuO bonds are connected to quasi two-dimensional networks forming 10A thick structural slabs parallel to the ab plane. Large displacement amplitudes of one oxygen atom along [1 0 0] (〈u 2 〉 = 0.032(5) A 2 ) and associated short repulsive oxygen-oxygen contacts [ OO ] = 2.682(4) A ) suggests a low-temperature structural phase transition. The average structure can be described as an intergrowth of tetragonal La2SrCu2O6 type structure slabs with CuO chains.

Growth of single crystals, thermal dependency of lattice parameters and Raman scattering in the Nd2-xCexCuO4-δ system

Abstract We report on the growth of Nd 2- x Ce x CuO 4- δ single crystals (0 x 2 O flux. Free separated crystals with maximum size of 5x8x0.15 nm 3 have been obtained. Magnetic AC susceptibility measurements show a sharp superconducting transition at temperatures up to 23 K. The temperature dependence of the lattice parameters has been measured by means of X-ray powder diffraction between 10 K ( a=3.9413(3) A , c=12.0290(18) A ) and 290 K ( a=3.9482(3) A , c=12.0590(18) A ). Room temperature Raman spectra reveal a new band at 320 cm -1 which is not observed in Nd 2 CuO 4 . Raman spectra of crystals with T c ranging from 7 to 22 K show a systematic intensity change of the broad band at 590 cm -1 .

Unusual behavior of the Gd ESR in single crystals of GdyY1−yBa2Cu3O6+x with x=0.1−0.8 and y=0.03−0.06: Evidence for magnetic interaction in the superconductors

The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of GdyY1−yBa2Cu3O6−x has been measured. In the insulating compound, with x∼0.1, and the superdconducting materials with 30 K < Tc < 80 K, the measurements were performed at X-band, 9.3 GHz, and Kα-band, 36 GHz, over a large temperature range above Tc. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual Tc of 70 K. The crystal field parameters are D = 3B02 = 1307 MHz, B04 = 3.014 MHz and B44 = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 0.005. These values change only slightly in the superconducting crystals.

Single crystal ESR studies on tetragonal YBa2Cu3O6+x

We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu2+ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.

Thermal expansion of Y2Ba4Cu7O15 in the temperature range 10–290 K, and comparison with YBa2Cu3O7 and YBa2Cu4O8

Abstract The orthorhombic lattice parameters of superconducting Y 2 Ba 4 Cu 7 O 15.28 ( T c = 85.5 K) are measured as a function of temperature (10–290 K) to a precision of 1 × 10 -4 on a low-temperature Guinier X-ray powder diffractometer by using an internal standard. The thermal expansion is anisotropic (( a 297 K − a 10 K )/ a 297 K = 17(2) × 10 -4 , ( b 297 K − b 10 K )/ b 297 K = 11(2) × 10 -4 , ( c 297 K − c 10 K )/ c 297 K = 34(2) × 10 -4 ) and intermediate between that of structurally related YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 . It is attributed to anharmonic thermal vibrations of the CuO 2 -planes and CuO single- and double-chains in the directions of vacant oxygen sites. No anomalous changes as a function of temperature are found within experimental resolution.