Suppression of superconductivity and the overdoped region in HgBa 2CuO 4+δ

Abstract

For the first time an overdoped, non-superconducting Hg-1201 phase was obtained by a high-pressure technique. The formal copper valence determined by iodometric titration was found to be +2.28(3). Superconducting Hg-1201 with T c = 35 K and v Cu = +2.20(3) was obtained by using the same technique and the same precursor, but which had been previously heat-treated. Further heat treatment of the sample increased T c up to 91 K and reduced v Cu to +2.13(3). We show that in the overdoped region of the Gh-1201 system the Cu valence can be changed by varying the precusor heat treatment. The instability of the overdoped phases makes it difficult to achieve the same results by heat-treating the as-prepared samples. A neutron powder diffraction study carried out on the overdoped non-superconducting sample resulted in a value of δ = 0.23(3). The formal copper valence obtained by iodometric titration and in the structural study can be reconciled if the presence of CO 3 2− defects on the Hg layers is assumed, or if a formal valence of −1 is adopted for the doping oxygen atoms.