Crystal structure of Y0.95Ca0.05CuO2.54: A new oxygen-deficient perovskite chemically and structurally related to the "123" superconductor

The crystal structure of Y0.95Ca0.05CuO2.54(1) has been determined by neutron powder diffraction at room temperature. The compound crystallizes with the symmetry of the hexagonal space group PG3/m and lattice parameters a = 6.1991(2), c = 11.2059(3) A. The configuration of the atoms in the unit cell may be alternatively described as an AgFe02-type structure with extra oxygen atoms in the copper plane, or as an oxygen-deficient perovskite of general formula AB03-x (x = 0.46) chemically and structurally related to the "123" superconductor oxide. In the unit cell the sequence of layers is ...[(Y3O6)(Cu2.43O1.62+ Cu0.57)(Y3O6)(Cu2.43O1.62+ Cu0.57)].... The yttrium and calcium atoms are mixed over three symmetrically independent sites. The coordination number of Y/Ca atoms is seven. The defective layer can be derived from a defective perovskite layer B02 in which rows of oxygen atoms are interleaved with double rows of oxygen vacancies, giving an oxygen stoichiometry of O2.67. The shifts of the oxygen atoms from their original perovskite positions and the hexagonal distortion of the lattice result in tetrahedral coordination around each Cu atom. The additional oxygen vacancies in the copper layers necessary to reduce the oxygen stoichiometry to O2.54 are assumed to be randomly distributed.