Compared structural behavior of Hg0.5Cr0.5Sr2CuO4.82 and Hg0.5Cr0.5Sr4Cu2O7CO2 superconductors

The structure of the 37 K superconductor Hg(0.5)Cr(0.5)Sr(4)Cu(2)O(7)CO(3) has been investigated by NPD, ED, HREM and compared to that of the parent '1201' 58 K superconductor Hg(0.5)Cr(0.5)Sr(2)CuO(5-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 stoichiometry and the existence of two types of copper polyhedra, namely CuO(5) pyramids and CuO(6) octahedra (50%-50%). But the large difference with the parent 1201 cuprate deals with the absence of chromium clustering, i.e., no local superstructure 'n(ap) x n(ap) x 2c' is observed in the carbonate contrary to the corresponding cuprate. Finally the XANES investigation shows that in both compounds chromium exhibits an oxidation state intermediate between IV and VI, so that superconductivity in these compounds is due to a charge transfer from chromium tetrahedra to copper layers. (C) 1998 Elsevier Science B.V. All rights reserved.