S. M. Green

New modulated structure in a Pb‐doped Bi‐Ca‐Sr‐Cu‐O superconductor

A new modulated structure has been observed in a Pb‐doped Bi‐Ca‐Sr‐Cu‐O superconductor (Tc,0=105 K) in addition to the incommensurate modulated structure reported by several researchers. This new modulation occurs in the b* direction and shows up as satellite spots to each of the original diffracted spots. Upon cooling to 88 K the diffraction spots due to the original modulation disappear, while the spots due to the new modulation remain. In the case of the undoped samples, the modulation does not disappear upon cooling to 30 K, which is below the Tc,0 of the sample (∼75 K).

Doping mechanism in Bi(Pb)-Sr-Ca-Cu-O superconductors

A model that explains the doping mechanism in Bi(Pb)‐Sr‐Ca‐Cu‐O superconductor is presented. X‐ray microanalysis reveals that the excess oxygen can be accounted for by the exact stoichiometry of the n=2 and 3 phases. We show that the presence of excess oxygen associated with the structural modulation in the compound is not sufficient to explain the hole concentration required by the transition temperature observed. Using a bond valence argument, we show that the effective valence of the Bi ion in the BiO bilayers is smaller than the expected value of +3, due to the incomplete oxygen coordination around the Bi ion. The model qualitatively explains the observations of lowering of Tc with Er (or Tm) doping or oxygen annealing. The role of Pb addition is suggested to be one of altering the thermodynamics and kinetics of formation of the n=3 phase.

Influence of oxygen on the structural stability and superconducting properties of ceramic (Bi,Pb)2Sr2Ca2Cu3Oδ

By post‐annealing ceramic samples in various partial pressures of oxygen, the structural stability and superconducting properties of (Bi,Pb)2Sr2Ca2Cu3Oδ are demonstrated to be sensitive to δ. The small decrease of Tc with increasing oxygen partial pressure is in agreement with previously reported results on Pb‐free samples and may be connected to an optimization of the charge carrier concentration. The collapse of the 2223 phase when post‐annealed above 700u2009°C in pure argon is also documented. It is suggested that this decomposition and the stabilization of the 2223 phase by Pb substitution are both related to the amount of oxygen contained in the (Bi,Pb)‐O layers which separate the Sr‐Ca‐Cu‐O perovskite units.

High-resolution electron microscopy of the c=30.5 Å and c=38.2 Å polytypoids in the Bi-Ca-Sr-Cu-O superconductor

We report results of high‐resolution structure imaging, image processing, and simulation of the c=30.5 A (Tc =80 K) and c=38.2 A (Tc =115 K) superconducting polytypoids in the Bi‐Ca‐Sr‐Cu‐O system. The increase in c parameter is due to the insertion of two layers of Cu‐O perovskite+Ca atoms in each unit cell. From the processed image and simulations, it appears that the central Cu layer in the c=38.2 A polytypoid may be oxygen deficient. In addition, the image simulation experiments suggest that the oxygen atoms in the Bi‐O layers are located at the face center of the Bi lattice.We report results of high‐resolution structure imaging, image processing, and simulation of the c=30.5 A (Tc =80 K) and c=38.2 A (Tc =115 K) superconducting polytypoids in the Bi‐Ca‐Sr‐Cu‐O system. The increase in c parameter is due to the insertion of two layers of Cu‐O perovskite+Ca atoms in each unit cell. From the processed image and simulations, it appears that the central Cu layer in the c=38.2 A polytypoid may be oxygen deficient. In addition, the image simulation experiments suggest that the oxygen atoms in the Bi‐O layers are located at the face center of the Bi lattice.

Effects of compositional variations on the properties of superconducting (Bi,Pb)2Sr2Ca2Cu3Oδ

The results of a systematic study on compositional variations within the Bi2−xPbxSr2Ca2Cu3Oδ superconducting system are reported. It is found that a secondary phase (or phases), likely composed of Pb‐Sr‐Ca‐Cu‐O, forms when x exceeds 0.35, indicating that this is an upper limit on the amount of Pb which replaces Bi. Varying the Sr:Ca or Bi:Ca ratio has dramatic effects on the resistive and magnetic properties. It is suggested that there is a very limited range of solution among the alkaline‐earth elements. Bi occupying Ca sites is probably deleterious for both Tc and the structural stability of the 2223 phase. Preliminary results on textured ceramic samples have yielded Jc (77 K) as high as 580 A/cm2 .

Grain boundaries and defects in superconducting Bi-Sr-Ca-Cu-O ceramics

Defects and structural interfaces in superconducting Bi‐Sr‐Ca‐Cu‐O have been characterized by transmission electron microscopy. The superconducting phase exhibits frequent variations in the stacking sequence (polytypoids). Dislocations, observed inside the grains, either introduce or accommodate the shear in the a‐b plane and the local composition fluctuations. In general, the grains exhibit a platelike morphology with the a‐b plane as the grain boundary plane. Grain boundaries along the short edge are generally disordered, whereas those near the long edge generally have a thin layer of the lower Tc polytypoid. Coherent intragranular boundaries are also observed.

Further evidence for the presence of c = 38. 2 A phase in a Bi-Ca-Sr-Cu-O superconductor

Results of high‐resolution electron microscopy, convergent beam electron microscopy, and x‐ray microanalysis on the high Tc phase are reported. Local inhomogeneities in the structure and composition have been observed along the c direction. Both c=30.56 A (8ap) and c=38.2 A (10ap) have been observed inside the same grain. The regions with the larger c parameter correspond to regions with higher Ca and Cu concentration suggesting that the range of transition temperatures observed (85 to 115 K) is due to the coexistence of such regions of c=38.2 A and c=30.56 A.

Phase formation in a Bi‐Sr‐Ca‐Cu oxide superconductor

The formation of different superconducting phases in the Bi‐Sr‐Ca‐Cu‐oxide system sensitively depends on the preparation procedures, particularly the firing temperatures. The tangled x‐ray powder diffraction patterns of coexisting Bi2Sr2CaCu2Oy and Bi2Sr2Ca2Cu3Oy have been completely identified. The basic unit cells are of approximate tetragonal symmetry. The lattice constants of the latter have been determined to be 5.40×5.40×37.09 A3.

Structure and composition of the 115 K superconducting phase in the Bi‐Ca‐Sr‐Cu‐O system

Results of convergent beam electron diffraction and energy dispersive x‐ray (EDX) microanalysis of Bi1.2Ca0.9Sr0.9Cu2Oy superconductor are reported. The structure is orthorhombic with a=5.63 A, b=5.4 A, and c=38.2 A. The c parameter is different from those reported by other workers and suggests that two extra perovskite units ap, where ap=3.82 A, have been introduced leading to c=10ap. EDX microanalysis shows that the Cu content of this phase is higher than that reported by other workers. It is suggested that the phase with c=38.2 A may be the higher temperature (115 K) superconductor.

Microstructure-property correlations in the Bi(Pb)-Sr-Ca-Cu-O superconducting system

The microstructure of solid‐state processed (Bi,Pb)‐Sr‐Ca‐Cu‐O ceramics was characterized using transmission electron microscopy techniques. A strong sensitivity of the transport properties to small deviations in the nominal Bi‐Ca ratio is evidenced. Significant differences in the microstructure are shown to correlate to the changes in the transport properties. It is suggested that the microstructure can be predicted by combining the results of resistivity, Meissner, and shielding experiments.