Dependence of T-c and transport properties on the Cu valence in HgBa(2)Ca(n-1)Cu(n)O2((n+1)+delta) (n=2,3) superconductorso

We have determined the oxygen content and the Cu valence by iodometric titiration of single-phase HgBa2Can-1CunO2O2(n+1+delta) (n=2,3), Hg-1212 and Hg-1223 superconductors postannealed under various conditions in either a reducing or an oxidizing atmosphere. Their lattice parameters, T-c, resistivity, and the thermoelectric power have been systematically investigated as a function of Ca valence. Hg-1212 exhibits a parabolic or ''bell-shaped'' relationship between T-c and the Cu valence similar to that reported for HgBa2CuO4+delta (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 T-c and the Cu valence confirm that the Hg-1223 sample with the highest T-c of 135 K is almost optimally doped. Both Hg-1212 and Hg-1223 as well as Hg-1201 exhibit their maximum T-c values at the same nominal Cu valence of approximately 2.20 which is also comparable to those for other high-T-c 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-O-delta charge reservoir supplies two holes is appropriate in the present Hg-12(n-1)n samples.