NEUTRON STRUCTURAL STUDY OF THE BI-MONOLAYER COMPOUND (BI0.5CU0.5)SR2(Y0.8CU0.2)CU2O7- THE ROLE OF EXCESS OXYGEN IN SUPERCONDUCTIVITY(DELTA)

Comparative neutron structural investigations are made on nonsupercond ucting (slow-cooled) and superconducting (liquid-nitrogen quenched; T( c)onset = 70 K and R = 0 at 20 K) samples of (Bi0.5Cu0.5)Sr2(Y0.8Cu0.2 )Cu2O7 + delta in order to examine the role of excess oxygen on the su perconducting behavior of this ''1212''-phase compound. Analysis of re fined structural parameters shows that the two main factors which infl uence the superconductivity in (Bi, Cu)-1212 are: (i) the extent of oc cupancy of excess oxygen at O(5), the 2(e) site located in between the two CuO2 pyramidal layers, and (ii) the length of the apical Cu(2)-O( 2) bond. The (Bi, Cu)-O monolayer plays the role of the ''charge reser voir'' quite effectively as reflected by the substantial increase in t he length of the apical (bridging) Cu(2)-O(2) bond due to depletion of excess oxygen in this layer. Surprisingly, vacancies at as many as 10 % of the O(1) site belonging to the CuO2 planes do not appear to disru pt the flow of current in the Cu(3d)-O(2p) planar network in the super conducting state. Relevant features of the structure of the (Bi, Cu)-1 212 phase, in particular the role of excess oxygen and its occupancy a t different sites, are discussed in the light of the available data on the isostructural (Pb, Cu)-1212 phase and the (La, Sr, Ca)3-Cu2O6+del ta phase superconductors.