COMPARED STRUCTURAL BEHAVIOR OF HG0.5CR0.5SR2CUO4.82 AND HG0.5CR0.5SR4CU2O7CO2 SUPERCONDUCTORS

The structure of the 37 K superconductor Hg0.5Cr0.5Sr4Cu2O7CO3 has bee n investigated by NPD, ED, HREM and compared to that of the parent '12 01' 58 K superconductor Hg0.5Cr0.5Sr2CuO5-delta (delta approximate to 0.12-0.18). This tetragonal oxycarbonate (a = 5.4812 Angstrom, c = 16. 1679 Angstrom) differs from the parent cuprate by the perfect oxygen s toichiometry and the existence of two types of copper polyhedra, namel y CuO5 pyramids and CuO6 octahedra (50%-50%). But the large difference with the parent 1201 cuprate deals with the absence of chromium clust ering, i.e., no local superstructure 'n(ap) x n(ap) x 2c' is observed in the carbonate contrary to the corresponding cuprate. Finally the XA NES investigation shows that in both compounds chromium exhibits an ox idation state intermediate between IV and VI, so that superconductivit y in these compounds is due to a charge transfer from chromium tetrahe dra to copper layers. (C) 1998 Elsevier Science B.V. All rights reserv ed.