Abstract
Lead was found to substitute for bismuth on cosubstitution of yttrium for calcium in nominal Bi4(Sr3Ca)Ca2Cu4O16. Variations with x in the lattice parameters, the oxidation parameter determined by iodometric titration, and the temperature dependence of resistance have revealed a correlation of the variation of Tc and c parameter versus x with a metal‐to‐semiconductor transition. It is argued that this correlation manifests a transition from itinerant‐electron to small‐polaron conduction. This conclusion is related to the observed shifts in c‐axis oxygen that appear to be associated with a transition from antiferromagnetic semiconductor to a superconductor in other copper‐oxide systems.