X-Ray absorption studies of bi-based superconductors

Abstract

X-Ray absorption (XANES and EXAFS) spectroscopic techniques have been applied to study the local structural environment of host and dopant cations in Bi-based high-T_c superconductors; the materials studied include the following: BaBiO₃, BaBi_{1 –x}Pb_xO₃, Ba_{1 –x}K_xBiO₃, Bi₂Sr₂CuO₆, Bi₂Sr₂CaCu₂O₈ and Bi₂Sr₂Ca₂Cu₃O₁₀. XANES spectroscopy has been used to determine the valence state of Bi, Pb and Cu in these materials. In BaBiO₃ the valence state of Bi is found to be close to IV both in the pure and doped material, and a disproportionation of Bi^IV into Bi^III and Bi^V is not indicated by analyses of these data. The valence state of Pb is also found to be IV in BaBi_{1 –x}Pb_xO₃. In Bi-based cuprate superconductors, the formal valence states of Bi and Cu are slightly higher than their stoichiometric values of III and II. EXAFS spectroscopy has been used to examine the local structure around Bi, Ba, Pb, Sr, Ca and Cu cations in these systems. In the BaBiO₃ system, doping with Pb and K results in a decrease in the metal–oxygen bond lengths. The results indicate that the local environment of all cations in the Bi–Sr–Ca–Cu–O system is characterised by substantial static disorder.