Oxidation state of copper and superconductivity in the HgBaCaCuO system

The oxidation state of Cu in HgBa2Can−1CunO2n + 2 + δ (n = 1−5) phases has been analyzed in terms of the bond valence sums. The totally mobile hole concentration (nH,t) of type-I CuO2 planes at the optimal level increases with the increasing number of CuO2 layers (n) per unit cell from 1 to 3, and the largest nH,t is observed to be 0.350 when n = 3, which just corresponds to the highest Tc in the Hg-based series. Thus the relationship between the maximum Tc for each member and the number n is similar to the correlation between the nH,t and the n.