Crystal and electronic structures of superconducting YSr2Cu3O6+x

In order to unveil the mechanism responsible for the large decrease of T-c (DeltaT(c) approximate to -30 K) found in YSr2Cu3O6+x with respect to YBa2Cu3O6+x, we compare structural and electronic properties of these two cuprates. We report X-ray, neutron diffraction, resistivity and ac susceptibility data and results of ab initio electronic structure calculations carried out within the local density approximation. Main structural differences between the two phases are: I) the absence of long CuO chains in YSr2Cu3O6+x possibly due to a large orthorhombic distortion predicted by calculations; 2) the strong compression of the CuO5 pyramids along the z-direction, which reduces the metallic character of the bond between the chain copper and the apical oxygen. This is expected to hinder the hole transfer from the CuO chains to the superconducting CuO2 planes, thus reducing the effective doping of the planes with respect to the optimum doping level. Both the disorder associated with short CuO chains and the reduced hole transfer would qualitatively account for the observed reduction of T-c.