SUPPRESSION OF SUPERCONDUCTIVITY AND THE OVERDOPED REGION IN HGBA2CUO4+DELTA

For the first time an overdoped, non-superconducting Hg-1201 phase was obtained by a high-pressure technique. The formal copper valence dete rmined by iodometric titration was found to be +2.28(3). Superconducti ng Hg-1201 with T-c=35 K and upsilon(Cu)=+2.20(3) was obtained by usin g the same technique and the same precursor, but which had been previo usly heat-treated. Further heat treatment of the sample increased T-c up to 91 K and reduced upsilon(Cu) to +2.13(3). We show that in the ov erdoped region of the Hg-1201 system the Cu valence can be changed by varying the precursor heat treatment. The instability of the overdoped phases makes it difficult to achieve the same results by heat-treatin g the as-prepared samples. A neutron powder diffraction study carried out on the overdoped non-superconducting sample resulted in a value of delta=0.23(3). The formal copper valence obtained by iodometric titra tion and in the structural study can be reconciled if the presence of CO32- defects on the Hg layers is assumed, or if a formal valence of - 1 is adopted for the doping oxygen atoms.