RELATIONSHIP BETWEEN RADIATION-INDUCED ORTHORHOMBIC-TETRAGONAL PHASE-TRANSFORMATION AND LOSS OF SUPERCONDUCTIVITY IN YBA2CU3O7

Under low-temperature ion irradiation, the superconductor YBa2Cu3O7 transforms into an insulator at relatively low doses approximately 0.04 displacement per atom. This change to the insulating state is accompanied by a crystallographic phase transition from the orthorhombic to tetragonal symmetry. There is no loss of oxygen and this phase transition has been attributed to the disordering of the oxygens in the plane of the chains. Results of electronic-structure calculations are presented in this paper which show that the ordering of the oxygen atoms in the plane of the chains plays a crucial role in the charge transfer from the CuO2 planes. Two models for the disordering of the oxygen atoms in the plane of the chains have been considered for this purpose. In each case, an alternate oxygen atom from the chains along the b axis is missing and is assumed to populate the originally vacant oxygen sites along the a axis, leading to a complete distruction of the chains and an identical population of the oxygen atoms along both the a and b axes in the irradiated sample. Our calculations show that this destruction of the chains under irradiation results in a drastic reduction of the electron transfer from the CuO2 planes to the planes of the chains, although the stoichiometry in oxygen remains unchanged. This reduction in the hole density renders the compound insulating. Our calculations also point out the vital role which the local coordination geometry of the chain-site Cu atom plays in controlling the charge transfer from the CuO2 planes. Displacement of the oxygen atoms from the chains to the originally vacant sites along the a axis in the two models of the irradiated crystal structure considered in the present work results in the transformation of the square-planar coordination of the chain-site Cu atom into a non-square-planar fourfold coordination. We find that it is this change in the coordination geometry that results in a drastic reduction of the hole-carrier density in the CuO2 planes.