E.L. Venturini

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.

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.

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.

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.

Orthorhombic‐tetragonal phase transition in high‐temperature superconductor YBa2Cu3O7

We have determined the effect of elevated temperature on the crystal structure and lattice parameters of the single‐phase high‐temperature superconductor YBa2Cu3O7 using high‐temperature x‐ray powder diffractometry. This compound is orthorhombic below about 550 °C (depending on oxygen partial pressure) and transforms to a tetragonal polymorph at higher temperatures. The reversibility of this transition depends on the oxygen partial pressure. An oxygen ‘‘hopping’’ mechanism with possible oxygen loss, which appears consistent with the known structural details, is proposed for the transition.

Synthesis and properties of Tl–Ba–Ca–Cu–O superconductors

We review the synthesis methods and properties of single crystal, powder and thin film TlBaCaCuO high-temperature superconducting (Tl-HTS) materials. With transition temperatures {ge}100K for several compounds, Tl-HTS materials present real opportunities for applications above 77 K. Experiments using (1) single crystals: determined precise structural parameters and identified the complex Tl{sup 1+}{minus}Tl{sup 3+} equilibrium model; (2) powders: studied the complex thermodynamic phase diagram; and (3) epitaxial films: have studied fundamental properties such as electron pair symmetry and the effect of controlled extrinsic defects on flux pinning strength, as well as providing the large-area surfaces required for device applications. {copyright} {ital 1997 Materials Research Society.}

Crystal structure of TlCa2Ba2Cu3O9

Abstract Attempts to prepare single cyrstals in the Tl-Ca-Ba-Cu-O superconductor system of phases having four or more consecutive copper oxide layers have produced small, single phase crystals of TlCa2Ba2Cu3O9 with a=3.8532 A and c=15.9223 A in P4/ mmm. The cyrstal has a superconducting transition at 117 K. The crystal structure is essentially that suggested from previous diffraction with three copper oxide layers which sandwich the calcium ions. The calcium sites contain a small amount of thallium and the structure refinement implies no oxygen vacancies.

Structure studies on Tl-2122 and Tl-2223 superconductors

Abstract Substantial differences in the superconducting transition, both in width and onset temperature, for the same structure-type have been observed in “pseudo-flux” grown crystals in the Tl-Ca-Ba-Cu-O system. These differences have been attributed to stoichiometry variations arising from cation site substitution, primarily the replacement of Ca 2+ with Tl 3+ between the Cu-O 2 layers. Several crystals belonging to the Tl-2122 (Tl 2 CaBa 2 Cu 2 O 8 with c ⋍ 29 A ) and the Tl-2223 (Tl 2 Ca 2 Ba 2 Cu 3 O 10 with c ⋍ 36 A ) structure-type have been examined by single crystal X-ray diffraction. The Tl-2122 phase has 15 to 28% Tl for Ca substitution while the Tl-2223 phase has only 8–9% substitution in the particular crystals examined. In addition, both phases also show a large variation in the Tl site occupancy (88 to 100%). Such cation disorder is consistent with the observed small differences in lattice parameters reported by various investigators in crystals of this system.

Pressure-induced crossover from long‑to‑short‑range order in [Pb(Zn1/3Nb2/3)O3]0.905 (PbTiO3)0.095 single crystal,

A pressure-induced crossover from normal Ferroelectric-to-Relaxer behavior has been observed in single crystal [Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3}]{sub 0.905}(PbTiO{sub 3}){sub 0.0095}, or PZN - 9.5% PT. Analogy with similar observations for other perovskites indicates that this crossover is a general feature of compositionally-disordered soft mode ferroelectrics. The Pressure-Temperature phase diagram has been also determined.

Growth of crystals and effects of oxygen annealing in the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O superconductor system

Abstract The growth of mm-sized crystals in the Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductor systems is discussed. The presence of polycrystals, which can be described as syntactic intergrowth crystals, is widely found. In the Tl-Ca-Ba-Cu-O s crystals of three distinct phases with superconducting transitions at 103, 106–108 and 112–114 K have been grown. In the Bi-Ca-Sr-Cu-O system crystals have been grown for two phases that exhibit transition temperatures at 80 and 92 K. Oxygen annealing is shown to substantially improve the superconducting properties in both systems.