C.C. Torardi

A New High-Temperature Superconductor: Bi2Sr3-x Cax Cu2O8+y

A new superconductor that displays onset behavior near 120 K has been identified as Bi2Sr3-xCaxCu2O8+y, with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 �, b= 5.414�, and c = 30.904 �. The structure contains copper-oxygen sheets as in La2CuO4 and YBa2Cu3O7, but the copper-oxygen chains present in YBa2Cu3O7 do not occur in Bi2Sr3-xCaxCu2O8+y. The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca2+ and Sr2+ cations between the adjacent Cu-O sheets; Sr2+ cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi2Sr3-xCaxCu2O8+y show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.

Crystal Structure of Tl2Ba2Ca2Cu3O10, a 125 K Superconductor.

There is now a new series of high-temperature superconductors that may be represented as (AIIIO)2A2IICan-1CunO2+2n where AIII is Bi or Tl, AII is Ba or Sr, and n is the number of Cu-O sheets stacked consecutively. There is a general trend toward higher transition temperatures as n increases. The highest n value for a bulk phase is three and is found when AIII is Tl. This compound, Tl2Ba2Ca2Cu3O10, has the highest transition temperature(∼125 K) of any presently known bulk superconductor. The structure of Tl2Ba2Ca2Cu3O10 has been determined from single-crystal x-ray diffraction data and is tetragonal, with a = 3.85 � and c = 35.9 �. No superstructure is observed, and the material is essentially twin-free. Electron microscopy in the Tl/Ba/Ca/Cu/O system has revealed intergrowths where n = 5; such regions may well be responsible for the superconducting onset behavior observed in this system at about 140 K.

Bulk Superconductivity up to 122 K in the Tl-Pb-Sr-Ca-Cu-O System

New high-temperature superconductors based on oxides of thallium and copper, but not containing barium, have been prepared. A transition temperature (Tc) of about 85 K is found for (Tl0.5Pb0.5) Sr2CaCu2O7 whereas (Tl0.5Pb0.5)Sr2Ca2Cu3O9 has a Tc of about 120 K. Both materials possess tetragonal symmetry with a = 3.80 �, c = 12.05 � for (Tl0.5Pb0.5)Sr2CaCu2O7, and a = 3.81 �, c = 15.23 � for (Tl0.5Pb0.5)Sr2Ca2Cu3O9. A structure refinement of the latter phase has been carried out with single-crystal x-ray diffraction data.

Superconductivity near liquid nitrogen temperature in the PbSrRCaCuO system (R=Y or rare earth)

Abstract Superconductivity close to the boiling point of liquid nitrogen has been discovered in the P-Sr-R-Ca-Cu-O system where R is Y or rare earth. The superconducting compound has been identified as Pb 2 Sr 2 (R,Ca) 1 Cu 3 O 8+ y and the structure of the R=Y compound has been solved by single-crystal X-ray diffraction methods. The structure consists of double CuO 2 sheets interleaved by (Ca,R), a unit which is common to most of the high- T c copper oxide superconductors. A new structural feature found in this system is a double PbO layer separated by a sheet of copper atoms. A non-superconducting oxide of approximate composition (Y,Ca)Sr 2 (Cu,Pb) 3 O 7− y formed as a second phase in some preparations has been also characterized; its structure is related YBa 2 Cu 3 O 7 .

Hole Density Dependence of the Critical Temperature and Coupling Constant in the Cuprate Superconductors

A bond valence sum (BVS) analysis was performed for the p-type cuprate superconductors. The superconducting critical temperature Tc versus in-plane Cu-O BVS correlation for copper is grouped into classes and subclasses. Only within a class or subclass for which the nonelectronic effect is constant does the variation of the in-plane Cu-O BVS reflect the corresponding change in the hole density nH of the CuO2 layers. This study strongly suggests that the Tc for every class or subclass of the superconductors is an inverted parabolic function of nH, and so is the coupling constant λ for Cooper pair formation.

Oxygen nonstoichiometry in copper-oxide based superconductors and related systems: Structure of nonsuperconducting Bi2Sr3−xYxCu2O8+y (x≈0.6–1.0)

Abstract Structural refinements of the new compounds Bi 2 Sr 3− x Y x Cu 2 O 8+ y in the range of x that gives insulating rather than superconducting behaviour ( x ≈0.6 and 1.0) have been obtained from single crystal X-ray and neutron powder diffraction data. Results indicate oxygen nonstoichiometry and suggest condensation into pairs or chains of the BiO atoms along the a axis in the BiO double layers. Chain-like formation can be found between the copper-oxygen sheets of all the orthorhombic superconductors including La 2 CuO 4 . XANES studies show that the oxidation state of copper (Cu 2+ is not affected by yttrium substitution for strontium. The mechanism of charge compensation appears to involve the insertion of oxygen, over the ideal O 8 composition, in (rather than between) the BiO sheets. These materials are orthorhombic and the lattice parameters for the subcell of the x ≈0.6 material are a =5.467(3), b =5.4407(6), c =30.434(5) A, space group Amaa, Z =4. The x ≈1.0 phase has a =5.467(2) b =5.426(1), and c =30.183(8) A. The structure is essentially the same as that of superconducting Bi 2 Sr 3− x Ca x Cu 2 O 8+ y . Electron diffraction patterns for the yttrium-containing phases show an incommensurate superlattice along the a axis as seen in the calcium analog. This is due to atomic displacements that are modulated along a with amplitudes in the c direction.

Crystal structure of TlBa2Ca2Cu3O9

Abstract Crystals of the 110 K superconductor TlBa2Ca2Cu3O9 have been grown and the structure refined from single-crystal X-ray diffraction data: a = 3.853(1), c = 15.913(4)A, space group P4 mmm , R = 0.055, R w = 0.057 . The structure contains triple copper-oxygen sheets separated by Ca ions which show ∼5% Tl substitution. These units alternate with single TlO layers. The existence of correlated atomic displacements within the thallium-oxygen sheets is again suggested by a disordered TlO arrangement in the average structure. The triple CuO sheets in TlBa2Ca2Cu3O9 are stacked directly above each other, whereas in Tl2Ba2Ca2Cu3O10, which contains double TlO sheets, alternate units are shifted by (a 1 + a 2 ) 2 .

Ferromagnetic R2Mn2O7 pyrochlores (R = DyLu, Y)

Abstract Pyrochlores of the type R 2 Mn 2 O 7 have been prepared and characterized where R is Dy, Ho, Er, Tm, Yb, Lu, or Y. All are semiconductors with activation energies ranging from 0.37 to 0.51 eV, which correlate very well with the electronegativity of R . Structural refinements on Er 2 Mn 2 O 7 and Y 2 Mn 2 O 7 confirm the pyrochlore structure and the stoichiometry. Highly anisotropic thermal motion is found for the rare-earth cation, consistent with its very unusual environment. These R 2 Mn 2 O 7 pyrochlores all possess magnetic ordering temperatures in the range 20 to 40 K. The susceptibility data above T c indicate dominant ferromagnetic interactions, i.e., positive Weiss constants. The field dependence data below T c are also suggestive of ferromagnetism. Nonetheless, the magnetic behavior is complex, and there may be no spontaneous moments in the absence of an applied field.

Superconducting Tl2.0Ba2.0CuO6+δ: A high resolution neutron powder and single crystal x-ray diffraction investigation

Abstract The structure of the orthorhombic form of Tl2Ba2CuO6+δ(Tc=90 K), synthesized in sealed gold tubes using BaO2, has been refined using high resolution neutron powder diffraction data collected at 4 K. No significant deficiency in metal stoichiometry could be inferred from the crystal structure analysis. The space group (Fmmm) used in this study gives essentially the same results as the alternative (Abma, Amaa) proposed in earlier work. The large amount of disorder that exists in the Tl2O2-slab is mirrored in the large thermal parameters for this structural unit compared to those for the CuO2 sheet. A site at ( built1 4 built1 4 built1 4 ) is occupied by an amount of oxygen to give overall stoichiometry Tl2Ba2CuO6.10, suggesting that oxygen non-stoichiometry plays an important role in producing high Tc superconductivity in this system. Analysis of single crystal X-ray diffraction data of tetragonal Tl2Ba2CuO6+ δ is totally consistent with the neutron powder diffraction results.

Superconducting and Magnetic Behavior in La2-xNaxCuO4

New phases of the type La2-xAxl+CUO4-y have been prepared where Al+ is sodium or potassium. The sodium phases are superconducting for x values from 0.2 to 0.5 at temperatures up to about 40 K. In addition, there are unusual magnetic properties below about 10 K that may be indicative of spin glass behavior. Phases of the type La2-xKxCuO4-y could only be prepared with x values up to about 0.1, and these phases are not superconducting above 4.2 K