Synthesis and crystal structure of Nd_1.57Ce_2.56Sr_3.87CuO_12.08

A new compound, Nd_1.57Ce_2.56Sr_3.87CuO_12.08, has been prepared and its crystal structure has been determined by the Rietveld method using a combination of neutron and X-ray powder diffraction data. The compound has hexagonal symmetry, P6̄2m (No. 189), with lattice parameters a = 10.18494(5) Å and c = 3.62891(3) Å. The coexistence of tetra- and trivalent rare-earth ions appears to be prerequisite for the formation of the compound. Three distinct crystallographic sites are occupied by the Ce, Nd, and Sr atoms. The Ce atoms prefer the octahedral sites, whereas the Sr atoms are located at sites with seven and nine oxygen neighbors. The Nd atoms occupy all three sites. The formal oxidation state of Cu atoms is found to be +1.47, resulting in a statistical coordination with either two or four oxygen atoms according to the valence state of copper ions, namely Cu¹⁺ and Cu²⁺. In either coordination, highly distorted environments around the Cu atoms are found. In contrast to the related compound, Pb_3+xSr_5-xCu_yO_12-d, the proposed structure of the title compound shows no evidence of a one-dimensional Cu-O chain along the c-axis.