Evidence for Multiple Defects in Tl₂Ba₂CuO_6+d and HgBa₂CuO_4+d Superconductors Observed by Neutron Powder Diffraction. O. Chmaissem¹, J.D. Jorgensen¹, D.G. Hinks¹, J.L. Wagner² B. Dabrowski³, ¹Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, ²Physics Department, University of North Dakota, Grand Forks, ND 58202, ³Physics Department, Northern Illinois University, Dekalb, IL 60115

Defect structures of Tl₂Ba₂CuO_6+d (Tl-2201) and HgBa₂CuO_4+d (Hg-1201) have been investigated by neutron powder diffration. At least two different defects are found to control the properties in each family of compounds. In Tl-2201, Cu substitution for Tl and interstitial oxygen atoms in the double Tl-O layer are found to control T_c and to result in variations in structure from tetragonal to orthorhombic for the same T_c. Maximum T_c is achieved for small oxygen defect concentration and the system is overdoped as oxygen is added. In Hg-1201, the dominant defect is oxygen in the Hg layer. By varying the oxygen partial pressure over a wide range during annealing, oxygen content can be used to both underdope and overdope the compound. Our neutron data suggest the presence of two coexisting and competing oxygen defects O3 and O4 in the Hg layer that contribute to doping. A crossover from O4 to O3 is observed in the region where samples have maximum T_c. This results in a "plateau" corresponding to the change in the defect chemistry with a constant T_c (at T_{c max}) for diffrent amounts of total defect oxygen. This could be interpreted as being due to an electronic topological transition when the van Hove singularity is crossed.

Work supported by the US Department of Energy, Basic Energy Sciences - Materials Sciences, contract W-31-109-ENG-38 and the National Science Foundation, Office of Science and Technology Centers, grant DMR 91-20000.