Sr-Cu-O system

In this chapter we have selected a number of case studies to show how atomistic lattice simulations can be used to investigate the structural and defect chemistry of a wide range of high-7 cuprates. Space limitations have necessarily restricted the number of examples to a small range of compounds, chiefly from the La Cu-O, Nd-Cu-O, Y Ba Cu O and Sr-Cu-O systems. 

IV. PREPARATION OF LARGE-GRAIN CRYSTALS A. Single Crystals of La-Sr-Cu-O System... 

Superconductivity in a bismuth-containing cuprate was first discovered in the Bi-Sr-Cu-O system by Michel et al. [9], whose Tc was as low as around 10 K. However, Maeda et al. [4] discovered extremely high Tc value above lOOK in the Bi-Sr-Ca-O system, attracting wide attention as a new high-temperature superconducting cuprate. Subsequent work has revealed the existence of a homologous series of superconducting oxides of the ideal formula... 

A higher temperature, Tc l 10K, phase in the Bi-Sr-Ca- Cu-O system has also been reported (78-80). This phase contains three infinite two-dimensional Cu-O layers. However, lead appears to play an important role in stabilizing this compound in phase pure form (75)(79)(80). Single crystals grown by eutectic melt growth techniques have been reported (77) but the phase purity of these crystals with respect to intergrowths containing various numbers of Cu-O planes is a concern. 

In favorable cases, superconductivity is a sensitive and useful screen for desired crystals. Near zero field microwave absorption (95) can be used to examine very small samples of only a few micrograms. This technique has value in the search for new superconducting phases. In early attempts to identify the superconducting phase in the Pb-Sr-Y-Cu-O system, both superconducting and non-supercon-ducting crystals were obtained. Individual crystals were examined for superconductivity using near-zero field microwave absorption. Then X-ray diffraction was used to establish the structure and stoichiometry of the superconducting phase. 

Within weeks after superconductivity was observed above 77 K in the Bi-Sr-Ca-Cu-O system, Sheng and Hermann (3) reported a record-high Tc value of 120 K for a Tl-Ba-Ca-Cu-O phase. The results were quickly confirmed (4)-(9) and the existence of a large family of thallium-containing superconducting oxides was revealed (10). 

Many of the high Tc ceramic superconductors can be prepared by similar solid state techniques. In the Bi-Sr-Ca-Cu-O system Bi2Os, SrCOs, CaCOs or Ca(OH)2, and CuO are often used... 

Figure 6 Single-phase region (open circles) for the 80 K superconductor in the Bi-Sr-Ca-Cu-O system (39).

A phase in the Bi-Sr-Ca-Cu-O system with a superconducting transition temperature near 110 K was apparent in many early mixed-phase samples (2X38). Superconducting onsets were often near 110 K although zero resistance was rarely achieved above 85 K. While chemical analysis, TEM, EDX, and singlecrystal X-ray diffraction suggested a composition Bi Ca Us... 

Bloomer, T.E., Golden, S.J., Lange, F.F., and Vaidya, K.J., Processing and characterization of single phase superconducting thin films in the Bi-Sr-Ca-Cu-O system from chemical precursors, Materials Research Society Fall (1989) Meeting, abstract M7.122. 

The high temperature superconductivity discovered in the Bi-Sr-Ca-Cu-O system was found to be related to a homologous series of compounds with an idealized formulation Bi2Sr2Ca iCu 02n+4 (n = 1 to 3 or more). The n = I, 2, 3 phases have Tc values of 10-20, 90 and 11 K, respectively. The presence of superstructure modulations which are, in general, incommensurate with the basic structure was first discovered by HRTEM and ED (Shaw et al 1988, Gai et al 1988). The periodicity of the modulation is found to be about 4.7 multiplied by the a-lattice parameter. The compounds can be prepared from solid state reactions of the component oxides in stoichiometric proportions and heating between 800-900 °C in air. 

Superconducting BSCCO films were produced in situ on 3" diameter sapphire substrates by plasma-enhanced halide LPCVD [266, 267]. Films consisting mainly of the Bi-2212 phase were deposited with metal halide precursors at 580°C under 0.1 Torr system pressure in the presence of a rf plasma. These films became superconducting at 70 K with = 2.5 x 10 Acm at 10 K. Plasma-enhanced halide LPCVD was also used to grow Bi-Sr-Ca-Cu-O/Bi-Sr-Cu-O superconductor-normal metal (S-N) heterostructures [259], HRTEM images showed the S/N interface to be atomically abrupt while variable temperature resistivity measurements gave T. = 75 K for the S/N heterostructure. 

The Tl (1223) phase of the pure Tl-Sr-Ca-Cu-0 system is a metastable superconductor. Among many samples prepared, only a few samples show a weak superconductivity or Meissner signal and many others show no superconductivity [7]. This is a reason the Tl-Sr-Ca-Cu-O system is called a metastable superconductor. When Tl is partially substituted by Pb, Sr by Ba, or Ca by Y, the system becomes a stable superconductor [13]. It is thought that the pure Tl-Sr-Ca-Cu-0 system is too doped to become superconductive. Reducing the carrier concentration by the substitution therefore leads to a superconductive state. Examples of an optimum composition are as follows ... 

Table 4.2-27 Stmctural data of superconducting compounds in the Bi—Sr—Ca—Cu—O system and of Pb substituted B12223 [2.15]...

Table k.2-28 Maximum Ti and a compilation of several values of the coherence lengths, London penetration depths and upper critical fields at T = 0 K for superconducting compounds of the Bi—Sr—Ca—Cu—O system, cont. 

---