Oxidation-Reduction of Pb sub 2 Sr sub 2 Ln sub (1-X) M sub x Cu sub (3-y) Ag sub y O sub 8

Cava, et al., have recently reported and subramanian, et al., confirmed a new family of high-T sub c oxide superconductors (T sub c ~ 70K) having the general formula Pb sub 2 Sr sub 2 Ln sub (1-x) M sub x Cu sub 3 O sub (8+delta), where Ln = La, Y and rare earths, and M = Ca or Sr. Subsequently the abbreviation 2213 will be used to denote this phase. This phase has the orthorhombic structure shown in Fig. 1. The basis of the new structure is double planes of corner shared CuO sub 5 pyramids, separated by planes of 8-coordinated Ln atoms. These double pyramidal planes sandwich PbO-CuO sub delta -PbO planes in an arrangement unique to this family of materials (center of Fig. 1A). In the stoichiometric material, i.e., x and delta = 0, it was proposed that the Cu atoms between the PbO layers are monovalent while those in the pyramidal sheets are divalent, leading to an average formal Cu valence of +1.67. As aliovalent ions (M sup (2+)), are substituted for Ln sup (3+), it was further postulated that the charge compensation occurs within the pyramidal CuO sheets only, thus raising the average formal valence of these Cu ions beyond +2 and inducing metallic conductivity and superconductivity. It was further proposed that when excess oxygen goes into the structure (delta>0), it is accommodated in the PbO-CuO sub delta -PbO sandwich.