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(Ba/Sr)O-Bi-layer ordering in the RE2(Ba/Sr)2GaCu2O9 superconducting cuprate

Ordering in the (Ba/Sr/Nd)O bi-layers (of "Rock- Salt" type structure) of the RE2(Ba0.65Sr0.20 Nd0.15)2GaCu2O9 high-Tc superconducting cuprate was investigated by electron microscopy and diffraction. Besides the average layered structure based on the tetragonal perovskite cell ap ´ap ´ cp and the meandering chain superstructure based on the orthorhombic 4ap ´2ap ´ cp 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 2ap ´ 2ap 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.

high-Tc ; superconducting cuprate ; crystal structure ; layer structure ; electron microscopy

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