Mechanism of Twin Formation During the Tetragonal to Orthorhombic Transformation In Yba 2 Cu 3 O 7-X
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MECHANISM OF TWIN FORMATION DURING THE TETRAGONAL TO ORTHORHOMBIC TRANSFORMATION IN YBa2Cu307-x MEHMET SARIKAYA, ILHAN A. AKSAY, AND RYOICHI KIKUCHI Department of Materials Science and Engineering; and Advanced Materials Technology Center, Washington Technology Centers, University of Washington, Seattle, Washington, USA 98195 The formation of twins in the YBa2Cu3 0 7 _x system has been studied theoretically on the basis of transmissionelectron microscopy observations. The shape of the tips of the twinned domains is determined from two elastic strains, one associated with the twin boundary and the other with the lattice distortion at the tip region. The tangent of the angle at the tip which was calculated from the model agrees with experimental observations sufficiently well. The growth mechanism of the twinned domains is analyzed with an energy minimization approach. The similarities of transformation characteristicsin this system to those of other systems are briefly discussed.
INTRODUCTION Many aspects of YBa 2 Cu307-x, and related oxides which exhibit superconductivity above 90 K are under study. From such studies, some of the phase transformation characteristics of the YBa 2 Cu307_,x phase have been well established. These are: (i) the superconducting form of the YBa 2 Cu307_x has an orthorhombic (0) structure with a space group Pmmm with a bulk b lattice parameter slightly larger than a and c • 3a 111;(ii) the orthorhombic phase is an ordered lattice of the tetragonal (T) phase in which oxygen atoms and vacancies preferentially order along 1010] and 1100] directions, respectively [2]; (iii) the T-- 0 transformation is second-order in character and takes place around 1000 K during cooling or heating 12-3]; (iv) as the T--,O transformation involves a shape change, strains are developed in the a-b plane (- 1%) 14]wlsich are accommodated by the formation of transformation twins 15-101; and (v) the formation of twins results in the development of a non-homogeneous structure with local variations in oxygen ordering and stresses in both single and polycrystalline samples I11] and, hence, causes variations in superconducting properties [Il], notably in T, and possibly in J, [12-13]. In conjunction with the above studies, the crystal structure of the materials has been characterized with bulk (e.g., x-ray diffraction and neutron diffraction) [1,4,14] techniques and the ordered oxygen domains by high spatial resolution transmission electron microscopy [15-16]. Hlowever, a detailed analysis of the microstructural features and their correlation with the phase transformation characteristics in the YBa 2 Cu.0 7 -, phase is still lacking. More notably, the formation of twins, their morphology and structure, and their influence on the T--O phase transformation are not fully understood 15,10,17-19]. The present work 1201 was undertaken to address some of these questions in relation to similar transformations [21-231 observed in other systems 120-221. In the present paper, specifically, we analyze the shape of the tip of twins
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