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AN ANTIPHASE BOUNDARY MODEL FOR

Bi Sr CaCu 0 2

2

2 8+x

J. REYES-GASGA, R. HERRERA, P. SCHABES-RETCHKIMAN, AND H.J. YACAMAN Apdo. Instituto de Fisica, Universidad Nacional Aut6noma de Mexico, Postal 20-364, 01000 Mexico, D.F., Mexico

INTRODUCTION The existence of a superconducting phase with a Tc onset near 1100C in the Bi-Sr-Ca-Cu-0 system was evidenced by Maeda et al, [1]. Tallon et al.

[21

have

described

Bi 21CaSr2Cu2 0 8+

several

with

Tc=

phases: 0

91 K

Bi 2. CaSrCuO 4+

and

with

Bi 2.1Ca Sr Cu 0

Tc= with

80°K; Tc=

105°K,named 2111, 2122, and 2223 respectively. All these phases present a modulated structure in the (100) plane and a distinct superstructure along the b-axis to which some attention has been devoted recently [3,41. In this 2122 plane of the of the (001) our analysis we present paper superconducting phase based on the antiphase boundary (APB) long-period superstructure theory [5].

LONG-PERIOD SUPERSTRUCTURES It is possible to correlate unambiguously the APB sequence observed in pecularities of the electron diffraction pattern. The HREM images with Fujiwara discussion for long-period superstructures present in alloys with cubic unit cell [5] will be described in this section. The period or length of the APB, called M, can be measured from the separation of split spots [5,61. The number M can be an integer, rational or an irrational number. It can be written as: M=

Ei (number of APB of length i) total number of APB

_ p q

(1)

where, therefore, p denotes the period of the sequence of APB. Notice that although the arrangement of APB depends on the M value, for a given M different arrangements of APB are possible. M can also vary continuously with concentration and temperature [6]. The APB diffraction patterns show four types of reflections (see e.g. [71). One type is the fundamental reflection corresponding to the lattice in fig. 1). The second type are superlattice of the structure ("A" the along the axis of reflections due to the APB's concentration superstructure ("B" in fig. 1). The third type are reflections due to the long modulation of the APB's ("C" in fig. 1); and the fourth reflection type are satellites due to a concentration or displacement modulation ("D" in fig. 1). The value of M is determined by the positions of the first order reflections of the third type. Fujiwara [5] shows that for these reflections, the diffraction patterns exhibit maxima at:

S=

n± (2m+1)/

2M; n,m integers.

(2)

the first order are formed at: e = n±(1/2M) and smaller therefore, secondary at t = n±(3/ZM). The intensity of successive orders decreases rapidly. Therefore, the diffraction patterns show that the basic spots are split into symmetrically situated satellites separated by a distance 1/2M left and right of the position of the basic reflections. Furthermore Van Tendeloo et al. [8] showed that the mean distance, D, of the superlattice reciprocal space is: reflections in

Mat. Res. Soc. Symp. Proc. Vol. 169. 01990 Materials Research Society

844

1/D= (1/a)-(2/(Ma)) where a is

the uni