THE RESISTIVE BEHAVIOR OF MIXED-PHASE YBCO AT TEMPERATURES ABOVE THE SUPERCONDUCTING PHASE-TRANSITION

The normal state resistivity of mixed phase YBa2Cu3O7-delta/Y2BaCuO5 ( 1:2:3/2:1:1) sinters are unusual in two respects. The resistivity, (rh o), increases far more rapidly with the proportion of the 2:1:1 phase (p) than is predicted by simple percolation theory, and its temperatur e dependence changes from rho(T) approximate to alpha + beta T for p m uch less than 1, to rho(T) proportional to 1/T close to the percolatio n threshold, p(th). This behaviour is compared with the predictions of a three-dimensional site percolation model. For low concentrations of the 2:1:1 phase, the 1:2:3 basal plane resistivity, rho(ab) proportio nal to T, dominates, while close to the percolation threshold the resi stive elements of the model are effectively in series, and the 1:2:3 c -axis resistivity, rho(c) proportional to 1/T, dominates. SEM studies indicate that the incorporation of the higher-melting point 2:1:1 phas e inhibits contact between the 1:2:3 grains, lowering the mean area of inter-grain contact When this effect is taken into account, the model is also capable of explaining the major part of the variation in the magnitude of the resistivity with concentration.