On the crystal structure of the “1201” superconductors Hg 0.5Bi 0.5Sr 2− xLa xCuO 5−δ

Abstract

Two compositions in the “1201” system Hg 0.5Bi 0.5Sr 2− xLa x CuO 5−δ, x = 0 and x = 0.5, have been studied by neutron diffraction, electron microscopy and X-ray absorption spectroscopy. Neutron diffraction gives evidence for a high disordering of the oxygen located in the Hg/Bi plane which is fully oxygenated. For the lanthanum-free sample, a slight oxygen deficiency is observed with oxygen vacancies (0.08 per cell) located in the basal plane of the copper octahedra. This feature is connected to the appearance of local superstructures observed by electron diffraction and high-resolution microscopy which tends to disappear after oxygen annealing and are enlarged after argon annealing. XANES spectroscopy at Bi L 3 edge shows a valence state close to +5 for bismuth in the lanthanum-free sample which is slightly reduced when lanthanum substitutes for strontium. In the same way copper K edge spectra show that lanthanum substitution leads to a decrease of the copper valency. The connection with superconductivity is discussed.