STRUCTURAL STABILITY OF BI-BASED CUPRATES

The relative structural stability of the Bi2Sr2Can-1Cu(n)O2n+4+delta compounds upon doping has been studied in terms of the lattice mismatch between the rock-salt (BiO)2 block and the perovskite block. Modification of the bond length in the rock-salt block by extra oxygen atoms or larger cations such as Pb is shown to be an important mechanism for the stabilization of the Bi compounds. The oxygen stoichiometric (delta = 0) and Pb free Bi cuprates are shown to be unstable. The minimum amount of extra oxygen to stabilize the n = 2 structure is estimated to be delta(min) almost-equal-to 0.08. It is also shown that Pb (in the + 2 state) which substitutes Bi, effectively increases the stability of the Bi compounds, noticeably the n = 3 phase, and favorably affects the kinetic process of phase formation. The minimum amount of Pb to stabilize the oxygen stoichiometric n = 2 compound, Bi2-xPbSr2CaCu2O8, is estimated to be x(min) almost-equal-to 0.32-0.33.