THE CRYSTAL-STRUCTURES OF DY2BA4CU7O14+DELTA AND ND2BA4CU7O14+DELTA - A HIGH-RESOLUTION POWDER NEUTRON-DIFFRACTION STUDY

The structures of Dy2Ba4Cu7O14+delta and Nd2Ba4Cu7O14+delta (247), which can be considered to be derived from alternate blocks of the LnBa2Cu3O7 (123) and LnBa2Cu4O8 (124) structures, have been refined using high resolution powder neutron diffraction data. Accurate structural parameters, determined by this oxygen-sensitive technique, are presented for these superconducting oxides. In both materials oxygen was found to be randomly distributed over the formally single chain and inter-chain sites of the type seen in 123 compounds; no crosslinking of the 124 double chains was observed. Notable differences in oxygen contents were found between the two samples; the neodymium sample exhibited a high average copper oxidation state as a result of considerable occupancy of both of the chain and inter-chain sites. The orthorhombic distortions of these materials are markedly reduced compared to those recorded for the corresponding 123 and 124 materials. A flattening of the copper-oxygen planes sandwiching the lanthanide site with increasing lanthanide size, as previously found in Ln123 and Ln124 compounds, is observed.