NEUTRON-DIFFRACTION STUDY OF THE LAYERED CUPRATE SR2-XND1+XCU2O6-Y

New layered cuprates Sr2-xNd1+xCu2O6-y, Sr2-xAxNdCu2O6-y (A=Ca, Ba) and Sr2Nd1-xLaxCu2O6-y have been synthesized and characterized by X-ray and electron diffraction. The structure of the extreme phase Sr2NdCu2O5.76 has been deteined by neutron diffraction. This oxide is a [2, 1]-member of the (ACuO3-x)m(A′O)n series of layered cuprates, i.e. corresponds to an intergrowth of single rock salt-type layers with double oxygen-deficient perovskite layers. However, the ordering of oxygen vacancies in the perovskite slabs is very complex and different from that observed in the classical [2, 1]-member La2CaCu2O6. The structure can be described by two extreme models (I) and (II). The first model which involves only CuO5 pyramids in the perovskite slabs is also characterized by hexagonal tunnels, whereas the second one which involves CuO4 groups and CuO5 pyramids (II) can be described as an intergrowth of "123"-slabs with single rock salt layers. Although they exhibit a mixed valency Cu(II)-Cu(III), these oxides do not superconduct. © 1990.