Single crystal X-ray diffraction study of the HgBa2CuO4+delta superconducting compound

A high precision X-ray diffraction analysis up to sin theta/lambda = 1.15 of a HgBa2CuO4+delta single crystal having a T-c of approximate to 90 K is presented. The cell parameters are a = 3.8815(4), c = 9.485 (7) Angstrom. The refinements indicate the existence of a split barium site due to the presence of excess oxygen in the mercury layer. The position of this excess oxygen might be slightly displaced from the (1/2 1/2 0) position. A 6% mercury deficiency is observed. Models, including mercury defects, substitution by copper cations, or carbonate groups, are compared. However, we obtain no definite evidence for either of the three models. A possible disorder of the Hg position, due to the formation of chemical bonds with neighbouring extra oxygen anions, could correlate to the refinements of mixed species at the Hg site. A low temperature single crystal x-ray diffraction study, and comparison of refinements for the same single crystal with different extra oxygen contents, are in progress to help clarify this problem.