COPPER-OXYGEN BOND LENGTH AND SELF-DOPING IN R2CUO4 (R = PR, ND, SM, EU, GD)

Bond-length mismatch between alternate layers of the T' R2CuO4 intergrowth structure introduces a stretching of the Cu-0 bond length in the CuO2 planes. The bond-length mismatch is relieved by self-doping. The character of the self-doping is probed by measurements of thermopower and lattice parameters for quenched and air-annealed samples. For R=Sm, Eu, or Gd, oxygen vacancies in the R/O2/R layers reduce the intralayer compressive stress and introduce antibonding sigma*(x2-y2) electrons into the CuO2 planes to relieve their intraplanar tensile,stress. In Nd2CuO4, a displacement of some oxygen to c-axis sites in the NdO layer permits essentially complete relief of the bond-length mismatch, and the annealed compound is slightly oxidized. The self-doping in Pr2CuO4 involves an internal redox reaction, and the possibilities of Pr(4+/3+) versus peroxide formation are evaluated. The data also indicate a decrease in Madelung energy with increasing Cu-0 bond length at the threshold for a transition in n-type transport from small-polaron to intermediate-size-polaron character.