We have determined the oxygen content and the Cu valence by iodometric titiration of single-phase HgBa2Can-1CunO2(n+1)+δ (n=2,3), Hg-1212 and Hg-1223, superconductors postannealed under various conditions in either a reducing or an oxidizing atmosphere. Their lattice parameters, Tc, resistivity, and the thermoelectric power have been systematically investigated as a function of Cu valence. Hg-1212 exhibits a parabolic or "bell-shaped" relationship between Tc and the Cu valence similar to that reported for HgBa2CuO4+δ (Hg-1201), while only a half of the parabola is observed for Hg-1223. The systematic variations in the resistivity and the thermoelectric power as well as the relationship between Tc and the Cu valence confirm that the Hg-1223 sample with the highest Tc of 135 K is almost optimally doped. Both Hg-1212 and Hg-1223 as well as Hg-1201 exhibit their maximum Tc values at the same nominal Cu valence of approximately 2.20 which is also comparable to those for other high-Tc cuprate superconductors. This coincidence of the Cu valence for optimally doped states suggests that the simple ionic model where each excess oxygen in the Hg-Oδ charge reservoir supplies two holes is appropriate in the present Hg-12(n-1)n samples.
|Number of pages||9|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 1997 Mar 1|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics