Hole concentration and T_c in Tl_2-yBa₂Ca_1-zY_zCu₂ O_8-x

The p-type copper-oxide superconductors all exhibit a maximum value of T_c at a saturation hole concentration p_s per Cu atom in the superconductive CuO₂ sheets. The maximum value of T_c varies from one structure type to another; it is not clear how P_s varies. After a brief review of this problem, the hole concentration in the system Tl_2-yBa₂Ca_1-zY_zCu₂ O_8-x, 0≤z<0.4, is determined with the assumption that the number of Tl-6s electrons per Cu atom does not vary appreciably with z on going from an antiferromagnetic semiconductor (z=0.3) to a superconductor with T_c=94 K (z=0.0). A P_s∼0.17±0.02 per Cu atom and a T_c∼(370p+35±7) K are obtained. The defects in the Tl_2-yO_2-x layer apparently trap out the Tl-6s electrons, probably at Tl(I) species, so the Tl-6s electrons contribute little to the electrical conductivity and the Seebeck coefficient, which were measured in sintered pelletized powders. An important expansion of the lattice a-parameter is noted on passing from the superconductive phase at z=0.2 with p=0.10±0.02 and T_c=72 K to a semiconductive phase at z=0.3 having, we assume, a p=0.02±0.02.