Models for Oxygen Ordering and Diffusion in Ba 2 Ycu 3 O x and Ba 2 ,Ycu 3-y M y O x (M = Fe, Co, Al, Ga)
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MODELS FOR OXYGEN ORDERING AND DIFFUSION IN Ba2 YCU 3 0x AND Ba2 YCu 3 _yMyOx (M = Fe, Co, Al, Ga) A. SANTORO National Institute of Standards and Technology, Gaithersburg, MD20899 INTRODUCTION One of the most important aspects of the structure of Ba2 YCu 3 Ox is the local arrangement of the oxygen atoms on the basal plane of the unit cell for compositions comprised between x = 6.0 and x = 7.0 (the "basal plane" is defined by the copper atoms forming the Cu-0 2 chains). The observation of diffuse scattering in single crystal x-ray and electron diffraction patterns shows that short range ordering of the oxygen atoms is present over a wide range of compositions, that it is confined to the basal plane and that its nature depends on the oxygen content and the method of preparation of the material examined [1,2,3]. It thus appears that the mechanism by which oxygen is added to, or eliminated from, the structure of Ba2 YCu 3Oxplays an important role in the arrangement of the oxygen atoms in the structure of materials with x < 7.0. In this paper we will discuss from a geometric point of view, models of vacancy formation and ordering in the pure compound, and possible configurations of the oxygen atoms in metal doped materials. These models take into account the existence of twinning in samples of composition 6.5 < x • 7.0, are consistent with the observed movement of the twin walls occurring when the oxygen stoichiometry is varied [4], and explain the symmetry and the composition of the metal doped compounds. Since the oxygen atoms relevant in this study are those located on the basal plane of the structure, in what follows we need to consider only the atomic configuration on this plane and we may ignore all the other atoms in the unit cell. TWINNING, TWIN BOUNDARIES, AND MODEL OF THE STRUCTURE FOR X = 7.0 A complete study of twinning in Ba2 YCU 3 0x has shown that all the twin operations theoretically consistent with a change of symmetry from P4/mmm to Pmmm are in general observed in single crystal x-ray photographs [4]. If we disregard the small twin obliquity (-l") caused by the difference of the aand b-parameters of the orthorhombic unit cell, the resulting twinned structure can be described as being made of domains joined together along the (110) or (1H0) planes, and in which the directions of the a- and b-axes are interchanged. Several structures of the twin walls have been proposed [3]. Depending on the model, the coordination of the copper atoms at the boundary is assumed to be: (i) two- and three-fold; (ii) distorted tetrahedral; (iii) pyramidal; (iv) pyramidal and square-planar; (v) pyramidal and octahedral. All these models, except (ii), can be excluded for one or more of the following reasons: (a) there is no evidence that the upper limit of the composition range depends on twin density; (b) sites along the a-axis on the basal plane have always been found empty for x = 7.0; (c) all the oxygen sites other than on the basal plane, are always fully occupied; and (d) 3fold coordinated copper is less favorable energ
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