Abstract
The structure of a Ca-rich composite-type crystal [A2Cu2O3]7+δ [CuO2]10 with A=Sr0.43Ca0.55Bi0.03 and δ=0.04 was determined by single-crystal X-ray diffraction using the 4D superspace group formalism. Thanks to the existence of a significant number of first-order satellite reflections, reliable results were obtained concerning the displacive modulation within each subsystem. A comparison is made with the accurate structural results obtained by Frost Jensen on similar composite crystals containing less calcium. A larger modulation amplitude is observed in our crystal. Weak interactions (Cu–O≃2.72 Å) are involved locally between the copper atoms of a [Cu2O3] layer and one oxygen atom of a [CuO2] layer, leading in some unit cells to a pyramidal coordination of copper. Such an interaction is not observed in the Ca-less-containing crystal of Frost-Jensen. However, the most important point of this study concerns the evidence for the first time of disorder phenomena inside the [CuO2] subsystem; this disorder was modeled with both Cu and O splitted sites. Two main configurations with realistic Cu–O bonds are privileged for the CuO4 squares inside the [CuO2] sublattice. The refinement results and the bond valence formalism give strong evidence that the A sites are occupied at random by Sr and Ca atoms, which display two types of coordination, 7 and 8, in the crystal.
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