Intergrowth of modulated structures in high Tc Bi-Sr-Ca-Cu-O superconductors

Abstract(Bi, Pb)-Sr-Ca-Cu-O ‘2223’ wires with varying Bi/Pb ratios and sinter times have been investigated by parallel physical measurements and analytical electron microscopy. The numbers of phases present, their relative amounts, compositions and structures are reported and correlated with the superconducting properties. In particular, the recently reported long period (010) modulation (2b*/q*=7.7) is found to be connected with the “2223” high Tc phase.

An important structure characteristic of the main phase in Bi-Sr-Ca-Cu-O superconductor is the modulated structure. In this paper, two types of one dimensional modulation, one type of two-dimensional modulation as well as other modulations have been observed by using TEM and discussed.

Bi2Sr2CaCu2Oy and Bi4Sr3Ca3Cu4Ow have been found to be superconductors with Tc of about 80 K. Their crystal structures are layered structures with tetragonal symmetry, which is characterized by the existence of Cu-O layers. The difference between their structures is that in Bi4Sr3Ca3Cu4Ow Ca occupies not only the Ca site in Bi2Sr2CaCu2Oy but also a part of the Sr site. In addition, the one-dimensional modulated structure along the b axis has been observed in Bi2Sr2CaCu2Oy, although no modulated structure has been reported in Bi4Sr3Ca3Cu4Ow. According to the previous works on electrical properties, when Ca is replaced by Y, Tc decreases and reaches zero around x = 0.6, corresponding to the decrease in hole concentration. The semiconductorlike behavior has been found in 0.6 < x ≤ 1.0 and 1.1 < v ≤ 2.0. Moreover, the antiferromagnetic ordering appears near x=1.0. Recently, we have found a characteristic domain structure in the modulated structure in 0.8 < x ≤ 1.0,

Pb addition makes easier to form the high Tc phase in the BSCCO system. However, Pb easily vaporized at high temperature. A controlled Pb potential method has been applied to grow the high Tc phase in films. Initially, films are deposited on cleaved MgO substrates using an rf magnetron sputtering system. These amorphous as-deposited films are heat treated in a sealed gold capsule along with a large pellet of Pb-added BSCCO. Details of the process and characterization of the films have been reported elsewhere (1). Films trated for 0.5h at 850° C contain mainly the low Tc phase with a small amount of the high Tc phase. Hawever, films treated for 3h at 850°C consist mainly of the high Tc phase. This film is superconductive with a Tc(zero) of 106K. The Pb/Bi ratio of the films, analysed by SEM- EDS, are 0.12 and 0.18 for heat tratment times of 0.5 and 3h, respectively. The present study investigates the modulated structures of these films using HREM.

Bi-based superconducting phases have been found to be members of a structural series represented by Bi2Sr2Can−1Cun−1On+4, n=1,2,3, and are referred to as 2201, 2212, 2223 phases. All these phases are incommensurate modulated structures. The super space groups are P2/b, NBbmb 2201, 2212 phases respectively. Pb-doped ceramic samples and single crystals and Y-doped single crystals have been studied by transmission electron microscopy.Modulated structures of all Bi-based superconducting phases are in b-c plane, therefore, it is the best way to determine modulated structure and c parameter in diffraction pattern. FIG. 1,2,3 show diffraction patterns of three kinds of modulations in Pb-doped ceramic samples. Energy dispersive X-ray analysis (EDAX) confirms the presence of Pb in the three modulated structures. Parameters c are 3 0.06, 38.29, 30.24Å, ie 2212, 2223, 2212 phases for FIG. 1,2,3 respectively. Their average space groups are all Bbmb.

The structures of various ordered, but non-periodic, systems have been investigated and exhibit features which can be directly described by means of a construction which the authors call the shift lattice , which is a simple generalization of the concept of the lattice. This paper is devoted to a description of the properties of the one-dimensional shift lattice and its Fourier transform. Its applications to the phases related to L–Ta 2 O 5 and some Bi 2 TeO 5 -related systems are outlined and its relation to the theory of modulated structures and their Fourier transforms is briefly discussed.