A New Layered Cuprate Structure-Type, (A1-Xa′X)14Cu24O4 1

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Phases crystallizing in a new structure-type with general formula (A1-xA′x)14Cu24O4 1 (A = alkaline earth metal, A′ = trivalent metal) and symmetry Cccm with an extended stoichiometry range have been found in studies of the SrYCuO, BaSrBiCuO and CaLaCuO systems. Single crystal X-ray studies on several crystals grown from different alkaline earth/metal oxide-cuprate melts reveal a common orthorhombic F-centered subcell of a=11.3 A ̊, b=13.0 A ̊ and c=3.9 A ̊. Superstructure is observed in crystals, leading to a 7-fold increase of the c-axis and a change in symmetry to space group Cccm. As in Ba2YCu3O7, the Cu atoms are found as CuO planes and linear CuO chains. Due to shear in the planes, half of the CuO squares share edges, producing CuCu zigzag chains, similar to the planes observed in CaCu2O3. In the linear CuO chains, the CuO squares share edges as well, leading to a short CuCu contact of 2.75Å. Experiments on ceramic samples indicate that the oxygen content is fixed and that the samples are semiconducting.



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