NEW EXTENDED DEFECTS IN THE SUPERCONDUCTIVE-123 OXYCARBONITRATE YCABA4CU5(NO3)X(CO3)1-XO11 - HREM STUDY

A HREM study of the 72 K superconductor YCaBa4Cu5(NO3)x(CO3)1-xO11 has been performed. One observes, for many crystals, a very regular contrast characteristic of the ''2a123 X b123 X 2c123'' superstructure corresponding to an ordered stacking of the rows Of CuO5 pyramids and of CO3 (NO3) groups along a over arrow pointing right, alternately. Contrary to YBa2Cu3O7, this compound does not exhibit any microtwinning in agreement with the pseudotetragonal character of its subcell. One observes three kinds of extended defects: (i) Intergrowths of other n members of the series (Y1-xCax)nBa2nCu3n-1(NO3)x(CO3)1-xO7n-3, involving different local superstructures with a = 3a123, 4a123, and 6a123; this suggests strongly the possibility to synthesize the members n = 3, 4, and 6. (ii) 90-degrees oriented domains similar to those previously observed for ''123'' type oxycarbonates, whose junctions have been interpreted. (ii) Intergrowths of the ''123'' oxycarbonitrate and the Sr2CuO2CO3-type structure. These defects are of capital important since they are the signature of the possible existence of a new series of oxycarbonates or nitrates, (Y1-xCax)2A2Cu4O9-(A2CuO2MO3)n for A = Ba, Sr; M = C, N with an original structure and which might be superconductive.