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 po sition of this excess oxygen might be slightly displaced from the (1/2 1/2 0) position. A 6% mercury deficiency is observed. Models, includi ng mercury defects, substitution by copper cations, or carbonate group s, 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, cou ld 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 c ontents, are in progress to help clarify this problem.