Pj. King et al., THE RESISTIVE BEHAVIOR OF MIXED-PHASE YBCO AT TEMPERATURES ABOVE THE SUPERCONDUCTING PHASE-TRANSITION, Phase transitions, 56(3), 1996, pp. 177-191
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.