THE GREAT FLEXIBILITY OF THE ROCK SALT LAYERS IN THE LEAD-BASED 1212-HIGH-TC SUPERCONDUCTIVE CUPRATES - THE OXIDES (PB, A)SR2(CA, LN)CU2O7-DELTA

A systematic investigation of superconductivity based on 1212-type cuprates with nominal compositions (Pb0.5A0.5)Sr2(Ca0.5Y0.5) Cu2O7-δ with A = Cu, Zn, Ni, Mg, Cd, Ca, Sr and (Pb0.7A0.3)Sr2(Ca0.5Y0.5) Cu2O7 with A = Sc, Fe has been carried out. It has been demonstrated that the different A cations enter into the structure, mainly in the rock salt type slabs, forming [(Pb1-xAx)O]∞ layers with an A content smaller than the nominal one. The study of the superconducting properties of these phases, and especially of their annealing properties at low temperature, shows that the critical temperatures of these phases, ranging from 30 to 90 K, are closely related to the nature of the A cations. The replacement of Y by Yb is also investigated. These results are discussed in terms of the size of the A cations, the oxygen nonstoichiometry, and the possible coexistence of the couples Pb(II)/Pb(IV) and Cu(II)/Cu(III). © 1993 Academic Press, Inc.