A 4D Structural Study of a Ca-Rich Composite-Type Crystal [A2Cu2O3]7+δ [CuO2]10 with Disorder Phenomena in CuO2 Sublattice

✍ Scribed by H. Leligny; B. Raveau; L.I. Leonyuk; V. Maltsev

• Publisher: Elsevier Science
• Year: 2002
• Tongue: English
• Weight: 465 KB
• Volume: 163
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2001.9333

✦ Synopsis

The structure of a Ca-rich composite-type crystal [A 2 Cu 2 O 3 ] 7؉ [CuO 2 ] 10 with A ‫؍‬ Sr 0.43 Ca 0.55 Bi 0.03 and ‫؍‬ 0.04 was determined by single-crystal X-ray di4raction using the 4D superspace group formalism. Thanks to the existence of a signi5cant number of 5rst-order satellite re6ections, 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 A s ) are involved locally between the copper atoms of a [Cu 2 O 3 ] layer and one oxygen atom of a [CuO 2 ] 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 5rst time of disorder phenomena inside the [CuO 2 ] subsystem; this disorder was modeled with both Cu and O splitted sites. Two main con5gurations with realistic Cu+O bonds are privileged for the CuO 4 squares inside the [CuO 2 ] sublattice. The re5nement 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.