Ik. Gopalakrishnan et al., NEUTRON STRUCTURAL STUDY OF THE BI-MONOLAYER COMPOUND (BI0.5CU0.5)SR2(Y0.8CU0.2)CU2O7- THE ROLE OF EXCESS OXYGEN IN SUPERCONDUCTIVITY(DELTA), Journal of superconductivity, 7(5), 1994, pp. 857-863
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.