ORAL SESSION 5C7: Microstructure/HTS Property Relation

Friday, Feb. 25, 9:00 a.m. – 11:00 a.m., Imperial Ballroom-East (Hyatt)

Chairs: F. Licci (Instituto MASPEC, Italy), M. Marezio (MASPEC-CNR/Parma)

5C7.1 Defect Chemistry at Optimal Doping in Layered Cuprate Superconductors

J.D. Jorgensen 1, D.G. Hinks 1, O. Chmaissem 1,2, S. Jain 1, and J.L. Wagner 3. 1 Materials Science Division and Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, IL 60439. 2 Department of Physics, Northern Illinois Univ., DeKalb, IL 60115. 3 Department of Physics, University of North Dakota, Grand Forks, ND 58202.

Presenting Author: J.D. Jorgensen

A remarkable feature of the chemistry of layered cuprate superconductors is the ease with which the composition with the maximum T_c is synthesized for the majority of the compounds. For example, YBa₂Cu₃O_6+x, when cooled slowly in air or oxygen, attains the oxygen content that gives the maximum T_c. A related observation is that it is often difficult to extend the doping beyond the maximum T_c using a single chemical variable. For example, YBa₂Cu₃O_6+x decomposes if the oxygen annealing pressure is increased to attempt to reach the over-doped state, the synthesis of La_2-xSr_xCuO₄ becomes difficult in the over-doped state, x³ 0.15, and the synthesis of under-doped Bi-2212 and Tl-2201, by removing oxygen, requires special annealing methods. These observations, while admittedly subjective, suggest that the defect chemistry may favor the optimally-doped composition and may be qualitatively different in the under-doped and over-doped regimes.

We have used controlled annealing methods, volumetric measurements of the oxygen uptake and evolution, and neutron powder diffraction to investigate defect chemistry as a function of doping in Hg-1201, Tl-2201, and Bi-2212. In all cases there is evidence for differences in the defect chemistry of the under-doped and over-doped regimes. These results have important implications for the synthesis of layered cuprates over extended doping ranges and also raise questions about what underlying physics causes the optimally doped composition to also be a "crossover" point for the defect chemistry.

This work is supported by the US Department of Energy, Office of Science - Materials Sciences, contract No. W-31-109-ENG-38, and the NSF Office of Science and Technology Centers, grant No. DMR 91-20000.

5C7.2 "Zero" Series of Multi-layered Copper Oxides, 01(n-1)n and 02(n-1)n:Synthesis and Properties Including Superconductivity

Maarit Karppinen 1,2 and Hisao Yamauchi 1. 1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan. 2 Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02015 Espoo, Finland.

Presenting Author: M. Karppinen

Among the superconductive and related multi-layered copper-oxide structures, M_mA_rQ_n-1Cu_nO_{m+r+2n± d} or M-m^(A)r^(Q)(n-1)n, containing ordered-oxygen-vacancy Q_n-1Cu_nO_2n blocks, rock-salt-type AO layers and M₀O_{m± d} "charge-reservoir" blocks with a layer sequence, -CuO₂-(Q-CuO₂)_n-1-AO- M_mO_{m± d} -AO-, the "zero" structures, A_rQ_n-1Cu_nO_2+2n or 0r(n-1)n, are special in the sense that they do not contain any charge reservoir. The present contribution describes the search strategy, reports the successful synthesis and discusses the unique chemical and physical characteristics of various interesting/novel 01(n-1)n and 02(n-1)n phases, including the systems LaCuO_3-d (0101) and (Ba,La)Y(Cu,Fe)₂O_5+d (0112), as well as the pristine, high-pressure synthesized Ba₂Ca_n-1Cu_nO_2+2n [0^(Ba)2(n-1)n; n = 2 ~ 4] phases and their c-axis expanded, water-containing derivatives obtained upon exposition to (humid) air.

5C7.3 Structural studies of new superconducting bismuthates (Sr,K)BiO₃

Catherine Bougerol-Chaillout 1, P. Bordet 1, S. Kazakov 2, J. Pshirkov 2, S.N. Putilin 2, E.V. Antipov 2, and M. Nunez-Regueiro 3. 1 Laboratoire de Cristallographie, CNRS, BP 166, 38042 Grenoble, Cedex 9, France. 2 Chemistry Department, Moscow State University, Moscow 119899, Russia. 3 CRTBT, CNRS, BP 166, 38042 Grenoble, Cedex 9, France.

Presenting Author: C. Bougerol-Chaillout

We have been able to synthesize a new bismuth-based oxide, SrBiO₃, and to induce superconductivity by K or Rb doping performed under high pressure. T_cmax is about 12K (for 45%-70% of K). These new compounds have a distorted perovskite-based structure. Powder synchrotron and neutron diffraction studies have shown that, as a function of K doping, the symmetry changes from monoclinic (space group P21/n) to tetragonal (space group I 4/mcm) and remains the same down to 4K. Above room temperature, SrBiO3 keeps the same structure until decomposition around 700°C, whereas the K-doped phases decompose around 300°C with a progressive loss of oxygen. An increase of T_c was observed when resistivity was measured as a function of pressure. Powder synchrotron studies indicate that, for K-doped samples, pressure induces an increase of the tetragonal distortion, as well as a change of the tilt angle of the octahedra until the pressure corresponding to T_cmax is reached. From bond valence sum calculations, the average Bi valence would be close to 4.7 v.u. at T_cmax. In the case of SrBiO₃, no noticable change was observed under pressure.

5C7.4 Control of Carrier Distribution in Layered Copper Oxides for Tailoring Magnetic Irreversibility

Hisao Yamauchi 1 and Maarit Karppinen 1,2. 1 Materials and Structures Laboratory, Tokyo Institute of Technology,Yokohama 226-8503, Japan. 2 Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02015 Espoo, Finland.

Presenting Author: H. Yamauchi

For tailoring high-T_c superconductors, i.e. layered copper oxides M_mA₂O_n-1Cu_nO_{m+2+2n± d} or M-m2(n-1)n:P/RS (P/RS = perovskite/rock-salt charge reservoir) with alternating superconductive Q_n-1Cu_nO_2n blocks and AO-M_mO_{m± d} -AO blocking blocks, in terms of carrier doping and superconducting property characteristics, the following steps are crucially important: (i) classifying the known-to-exist layered copper-oxide structures, (ii) evaluating the crystallographical and chemical factors to control the oxygen non-stoichiometry and the charge distribution, (iii) establishing the techniques for probing the charge/carrier distribution, and (iv) observing experimentally the dependence of superconducting properties on the carrier distribution in various M-m2(n-1)n:P/RS phases. Based on the aforementioned procedure, the relationship between the irreversibility-field characteristics and the overall oxidation state and/or the carrier distribution is addressed with relevant experimental data on the Cu-12(n-1)n:P (n = 2 ~ 4), Bi-2212:RS and Hg-1223:RS phases as well as on the 0223 phase of the Ba-Ca-Cu-O system and its c-axis expanded "watery"derivative, (H,C)-m223 (m » 5).

5C7.5 Relationships between the structural and microstructural features of the Hg based single crystals and their superconducting properties

A. Maignan, A. Daignere, D. Pelloquin, V. Hardy, A. Wahl, M. Hervieu, and B. Raveau, Laboratoire CRISMAT, UMR CNRS ISMRA 6508, 6 bld Maréchal Juin, 14050 CAEN Cedex, France

Presenting Author: A. Maignan

The stabilization of baryum based mercury cuprates by foreign elements M in the mercury mixed layer [Hg_1-xM_xOd ] has allowed us to synthesize numerous Hg_1-xM_xBa₂Ca_n-1Cu_nO_2n+2+d cuprates (M=Ti, V, Cr, Mn, Cu, Nb, Mo, Ru, W, Bi, Ce, Pr, Nd; 1£ n£ 3). Moreover, for M=Ti, V, Cu, Cr, Mo and Bi, single-crystals have been grown. Their structural study reveals a random distribution of the M cation in the (Mg,M) layer where the M cation is also shown to increase the oxygen content. By post annealing treatments, underdoped or optimally doped crystals (M=Cu) and overdoped ones (M=V, Bi) have been obtained. From the transport properties of 1201 with T_c=96K, a ratio r _c/r _ab=26 at room temperature is obtained emphasizing the moderate anisotropy of this phase. Similarly to other superconducting cuprates, the magnetization (M) measurements show the existence of M steps consistent with a first order transition.

5C7.6 Understanding Cuprates with the Highest T_cs

T.H. Geballe and B.Y. Moyzhes, Laboratory for Advanced Materials/Applied Physics Department, Stanford University, Stanford, CA 94305-4045

Presenting Author: T.H. Geballe

There has been a large factor of 4 increase in T_c above that found in the original "214"structure discovered by Bednorz and Mueller. We believe there are two approaches for understanding the superconductivity found in the highest T_c superconductors. One is to assume that the pairing is confined to the blocks of CuO₂ layers, as is the case for most models which are currently being investigated. We believe a more plausible approach is to consider that the large enhancement of Tc is due to additional pairing mechanisms away from the CuO2 layers. We cite chemical and structural considerations, and physical evidence for the existence of these additional pairing interactions that can account for enhanced Tcs above 160K and gaps above 1000K in the highest Tc superconductors. In some sense the complex unit cells are natural realizations of the old "sandwich" superconductivity advocated long ago by Ginzburg and Bardeen and their collaborators (although there are significant differences in the pairing mechanism).

The charge reservoir layers in the highest T_c cuprates contain layers of Tl, Bi or Hg. These are heavy atoms with 6s electrons and are known from chemistry to be negative-U centers in 6s¹ oxidation states (6s¹ + 6s¹ = 6s⁰ + 6s²) which can be additional sources of pairing energy. In the Hg cuprates strong additional pairing can result from real or virtual electron pair bond Hg⁺² + Hg⁺² + 2e ® Hg⁺¹ – Hg⁺¹ which is known to be the ground state of monovalent Hg. Such a view is consistent with d-wave symmetry and with the surprising amount of disorder in the HgO layers. It is also consistent with the unusually large dT_c/dP found in the Hg cuprates, and with nuclear resonance and optical conductivity data in the literature.

If an additional pairing interaction also exists in the double chains of the "248" structure it can account for the unusual pressure dependence, T_cmax = 108K (vs 95 for the "123" cuprate) and also the large temperature independent anisotropy in the in-plane penetration determined by optical studies.