(Ba/Sr)O-BI-LAYER ORDERING IN THE RE₂(Ba/Sr)₂GaCu₂O₉ SUPERCONDUCTING CUPRATE

Parts of this paper have been presented at "2^nd Croatian Symposium on Application of Electron Microscopy in Life Sciences and Materials Science'', Zagreb, October 4, 1996.

OGNJEN MILAT and IVICA AVIANI

Institute of Physics of the University, Bijenička 46, HR-10001, Zagreb, Croatia

Received 14 February 1997

This paper is dedicated to Professor A. Bonefačić on the occasion of his 70^th birthday

Ordering in the (Ba/Sr/Nd)O bi-layers (of "Rock-Salt" type structure) of the RE₂(Ba_0.65Sr_0.20Nd_0.15)₂GaCu₂O₉ high-T_c superconducting cuprate was investigated by electron microscopy and diffraction. Besides the average layered structure based on the tetragonal perovskite cell a_p×a_p×c_p and the meandering chain superstructure based on the orthorhombic 4a_p×2a_p×c_p cell in the GaO-layers, an additional ordering was found in the pairs of (Ba/Sr/Nd)O-...-(Ba/Sr/Nd)O layers. We propose a model for ordering of 3Ba and 1Sr cations on a two-dimensional 2a_p×2a_p lattice cell in each (Ba/Sr/Nd)O layer. This compositional intra-layer ordering is accompanied by the inter-layer plane stacking disorder. The 4-fold symmetry of the average tetragonal structure is preserved, while the translation parameters in the (a,b) plane are doubled. This local ordering in the RS-type bi-layers hardly affects the symmetry of the basic structure; however, it may act as a source of weak anisotropy in the CuO₂-layers.

UDC 538.911, 538.945
PACS 61.10.My, 61.16.-d, 74.72.-h

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