HREM STUDY OF THE 30 K SUPERCONDUCTOR BI2SR4CU2CO3O8

The structure and the microstructure of a new high Tc superconductor Bi2Sr4Cu2CO3O8 have been studied by high-resolution electron microscopy. The [110] orientation allows us to image the layers stacking, and the [010] calculated images allow us to interpret the contrast. The oxycarbonate Bi2Sr4Cu2CO3O8 exhibits a modulated structure; the mean value of the component of the modulation vector along b → is 8.85 but this periodicity is not constant. The relative arrangement of the double bismuth layers [BiO]2 involves the periodic compression of the intermediate layer, resulting in a strong variation of the contrast. The existence of additional slices, [Sr2CuCO3O2] or [Bi2Sr2CuO6], suggests the existence of a large family which can be formulated [Bi2Sr2CuO6]n [Sr2CuCO3O2]n′; n′ = 1 to 5, and n = 1 and 2 have been observed. The existence of a shearing mechanism along c → involves the connection of layers of different natures through the boundary; this mechanism is not only observed as isolated defects but can lead to the formation of a new “collapsed oxycarbonate” phase. Other structural features such as the micalike morphology and the existence of 90° oriented domains are discussed. © 1994 Academic Press, Inc.