X-RAY SINGLE-CRYSTAL ANALYSIS OF HGBA2CA2CU3O8+DELTA AND INFLUENCE OFOXYGEN STOICHIOMETRY ON THE SUPERCONDUCTING PROPERTIES

X-ray diffraction analyses of good-quality single crystals of HgBa2Ca2 Cu3O8+delta synthetized by a closed-vessel technique at low pressure, have lead to more definite results with respect to previous structural studies. Samples with T-c=135 K showed a Cu occupancy of 16.1% on the Hg site at the origin of the unit cell. Excess oxygen appears to be p resent only in the basal plane at the interstitial site 1/2, 1/2, 0 wi th an occupancy delta = 0.190 +/- 0.015. This compound has the shortes t copper oxygen apical distance within the mercury family (2.696(3)Ang strom) together with an almost complete planarity of the CuO2 planes. Variations in the excess oxygen content upon different heat treatments were analyzed by thermogravimetry and magnetic-susceptibility measure ments. A correlation between delta and T-c could be derived, showing i n particular that superconductivity exists down to a very low intersti tial oxygen content.