High Resolution Transmission Electron Microscopy Study of Defect Structures in Mixed-Phase YBaCuO
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HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY STUDY OF DEFECT STRUCTURES IN MIXED-PHASE YBaCuO M. Fendorf, E. Kvam, and R. Gronsky, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720
ABSTRACT Mixed-phase superconducting YBaCuO is characterized by means of high-resolution transmission electron microscopy. This technique reveals that such material contains numerous crystallographic defects of various types. Several of these are described in detail, and their origins discussed.
INTRODUCTION It is now known that superconducting YBaCuO exists in at least three well characterized phases of differing copper content. [1,2] Due to the cation ratios (Y:Ba:Cu) these compounds are commonly referred to as the "1-2-3", "1-2-4", and "2-4-7" phases. When YBaCuO crystals are synthesized in practice, an intergrowth of more than one structure frequently occurs in order to accommodate local deviations from these ideal stoichiometries. The superconducting materials examined in the present study were initially prepared as pure granular YBa2Cu4010 (" 1-2-4"), and then "partially converted" to YBa2Cu307 (" 1-2-3") by annealing at constant temperature and oxygen pressure. X-ray diffraction studies indicate that the resulting material contains both the "1-2-3" and "1-2-4" phases, as intended. High-resolution Transmission Electron Microscopy (TEM) has been utilized to study partially converted YBaCuO. TEM specimens were prepared by crushing the material to a fine powder and by ion milling, as described in detail elsewhere. [3] The material was seen to be a mixture of the "1-2-3" and "1-2-4" phases, in agreement with the x-ray data. High-resolution TEM also showed that a variety of crystallographic defects are present in the partially converted samples, as expected when a mixture of phases with differing lattice parameters is present. The defects observed were volumetric, planar, and linear in nature. RESULTS AND DISCUSSION The volumetric defects found in partially converted YBaCuO were intergrowths which consisted of essentially random alternation in stacking of differing phases (i.e. polytypoid behavior), where "1-2-3" and "1-2-4" structures both appeared without any short-period sequential order. Regions containing such structural mixtures were generally inhomogeneously strained, as evidenced by the variations from axial orientations in highresolution micrographs (see Figure 1). Interestingly, no Cu or Cu-complex oxides were seen on a 5gtm scale of observation, (c.f. grain size of about ltm) meaning that if such phases exist, they must be coarsely distributed in comparison with the grain size. The presence of alternate Cu-containing phases has been variously reported for granular YBaCuO materials [4,5]. These may, in fact, have been present here, but unseen due to the working magnifications employed. The most common platiar defects observed in the mixed-phase YBaCuO were extrinsic stacking faults of the type R = 1/6[031], described by Zandbergen et al. [6] Such stacking faults
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