Oxidation State of Cu and Superconductivity in Bilayer T1-Based Cuprates

The oxidation state of Cu in the homologous Tl₂Ba₂Ca_n-1Cu_nO_2n+4+δ(n = 1 - 4) series has been analyzed in terms of the bond valence sums. It is found that the totally mobile hole concentration n_H,t of the type-I CuO₂ planes at the optimal level increases with increasing number of CuO₂ layers per unit cell from 1 to 3, and the largest n_H,t is observed to be 0.474 when n = 3, which just corresponds to the highest T_c in the bilayer T1-based materials, and then it decreases with the increase of n. It is indicated that the correlation between the maximum value of T_c and n is similar to that between n_H,t and n.