C-AXIS OXYGEN IN COPPER-OXIDE SUPERCONDUCTORS

Exploration of the roles of c-axis oxygen in the copper-oxide superconductors has revealed a new type of tetragonal-orthorhombic transition in the system R2-zCezBa2-yLyCu3O8+x(R=Nd or Gd, L=La or Nd) and one probably due to cluster rotation in fluorinated La2-yDyyCuO4-xF2x having the T* structure with interstitial F ordered in the rocksalt layer. The former is due to c-axis ionic displacements that relieve a compressive stress on the (Ba,L)O rocksalt sheets, the latter a tensile stress on the LaO rocksalt sheets. In the YBa2Cu3O6+x structure, the c-axis oxygen participates in oxygen diffusion in the superconductively inactive layers, they act as gates for stabilizing holes in Cu(1)Ox planes versus CuO2 sheets, and they modulate the width W of the x2-y2* conduction band. These modulations are shown to be capable of changing the bandwidth from W<U to W>U and hence to provide a rationalization for the nonsuperconducting metallic copper oxides containing copper in a formal valence state greater than 2+. As modulators of bandwidth, their displacements offer electron-lattice interactions for pairing superconductive holes. In the n-type superconductors, interstitial c-axis oxygen perturbs the x2-y2* band so as to introduce localized states at the band edges that suppress superconductivity. © 1990 The American Physical Society.