TL0.5PB0.5SR2CUO5 - A NONSUPERCONDUCTOR DESPITE AN OPTIMUM HOLE CONCENTRATION IN THE CU-O SHEETS

Doping of La3+ for Ba2+ in TlBa2CuO5 leads to superconductivity in the material TlBa1.2La0.8CuO5 (with T(c) almost-equal-to 52 K). An increase in the Cu-O in-plane bond distance [1.9240 (1) angstrom] from that in TlBa2CuO5 (1.915 angstrom) is consistent with a lowering of the average hole concentration, p, in the Cu-O planes from 1.0 to approximately +0.4. We have doped Pb4+ for Tl3+ in TlSr2CuO5 to give an average hole concentration of +0.53, but find that Tl0.5Pb0.5Sr2CuO5 is not superconducting down to 5 K. The structure is similar to TlBa2CuO5 and TlSr2CuO5, belonging to the space group P4/mmm. Structural refinement using neutron powder diffraction data collected at 300 K show that the planar lattice constant [a = 3.7309 (3) angstrom] is considerably smaller than that for TlBa1.2La0.8CuO5. This leads to the substantially smaller Cu-O in-plane bond distance of 1.8654 (15) angstrom. The atoms in the Tl/Pb-O(3) planes are again disordered, and both the cation and the oxygen site occupancies are close to stoichiometry. This stoichiometry and the Tl/Pb oxidation states suggest that the hole concentration is close to the optimum value for these materials. Several reasons for the absence of superconductivity are discussed.