Crystal chemistry of CuBa2Can-1CunOy (n=4, 5, 6) superconductors

The crystal structures of Cu-based superconductors CuxBa2Can-lCunOy and (Cu,C)BawCan-1CunOy with n=4, 5, 6 have been investigated using intensity data collected on the single-crystal X-ray four-circle diffractometer. The Cu-site occupancy x in the deficient Cu(1)O-2 sheet between two BaO layers increases From 0.62 to 0.73 accompanied by a decrease in the c-axis length in CuxBa2Ca3Cu4Oy. The thickness of the charge reservoir-layer block decreases with decreasing c-axis length from 8.421(4) Angstrom for c=18.0016(39) Angstrom to 8.373(3) Angstrom for c=17.9434(8) Angstrom in CuBa2Ca3Cu4Oy, and from 8.405(2) Angstrom for c=21.1546(14) Angstrom to 8.34(1) Angstrom for c=21.0784(15) Angstrom in CuBa,Ca,Cu,O,, respectively, while the thickness of the superconducting-layer block unchanged in both series. From the results of the structure refinement and chemical analysis, we propose a general formula (CuxC1-x-delta)Ba2Can-1CunOy for these nonstoichiometric compounds, not only for the CuxBa2Can-1CunOy series but also for the carbonate-containing (Cu,C)Ba2Can-1CunOy compounds, where delta equals the vacancy content in the defect site. (C) 1997 Elsevier Science B.V.