Evidence for two competing defects in HgBa2CuO4+δ

The defect structure of HgBa2CuO4+delta has been investigated by neutron powder diffraction. By varying the P(O-2)'s over a wide range during annealing, the oxygen content can be modified to both underdope and overdope the compound. Our data give evidence for the presence of at least two defects that contribute to doping. The competition between the two defects results in a "plateau" in the region of maximum T-c. This plateau can be explained by a change in the defect chemistry that results in a constant T-c (at T-c max) for different amounts of total defect oxygen. This could be interpreted as being due to an electronic topological transition when the van Hove singularity is crossed. (C) 1998 Elsevier Science B.V. All rights reserved.