OXYGEN CONFIGURATION AND DISORDERING IN THE SUPERCONDUCTOR (HG0.7TL0.3)(2)BA-2(Y0.8CA0.2)CU2O8+DELTA

The structure of (Hg0.7Tl0.3)(2)Ba-2(Y0.8Ca0.2)Cu2O8+delta with a T-c of 65 K was determined by neutron powder diffraction. Rietveld refinem ent based on space group I4/mmm showed a highly disordered atomic arra ngement on double HgO1+delta/2 sheets. Both Hg/Tl and 03 atoms on the double sheets had apparently large isotropic atomic displacement param eters, which results from incomplete modelling of their disordered con figuration. The occupation factor, g, of the O3 site split into four p ieces, was reported to be 0.194 in Hg2Ba2YCu2O7.55 whereas g(O3) was v ery near to 1/4 in (Hg0.7Tl0.3)(2)Ba-2(Y0.8Ca0.2)Cu2O8+delta (delta ap proximate to 0). Such negligible oxygen deficiency is evidently due to partial substitution of Tl3+ ions for Hg2+ ions. Distances between ap ical oxygen, O2, and cations surrounding it are discussed in detail fr om a crystal chemistry point of view. A bond-valence sum of +2.084 for Cu in the present oxide supports the idea that it is underdoped with hole carriers. (C) 1997 Elsevier Science B.V.