Neutron powder diffraction of the superconductor TlBa2Ca2Cu3O8+δ with different maximum TC values (TC = 120-132 K)

The crystal structure of the high-TC superconductor TlBa2Ca2Cu3O8+δ with different maximum TC values (∼132, 126 and 120 K) is investigated by neutron powder diffraction measurements. The refined values of occupancy for the excess oxygen(δ) located in interstitial positions on the Tl-O layer, O(4), are 0.775(6), 0.751(16) and 0.813(25) for the samples with a maximum TC of ∼132, 126 and 120 K, respectively. It is found that the Cu(2)-O(3) bond lengths of samples with higher TC are shorter, and the value of this bond length for the Tl-1223 sample with a TC of ∼132 K is almost equal to that for Hg-1223 with a maximum TC of ∼135 K. It is also found that the flatness of the CuO2 planes increases with increasing TC. It appears that the structural features of the CuO5 pyramids are among the factors contributing to the higher value of TC in cuprate superconductors