Structure of the new "1201" lead cuprate superconductor

The structure of the newly discovered "1201" lead cuprate superconductor (Pb, Cu) (Sr, La)2CuO5-δ with Tc=27.5 K at onset and a shielding fraction 38% at 5 K and 10 Oe, has been determined using neutron powder diffraction. The structure is similar to the other 1201 materials TlBa1.2La0.8CuO5 and Tl0.5Pb0.5Sr2CuO5 (where the former superconductors with a Tc of K and the latter is not superconducting), belonging to the space group P4/mmm. The doping scheme in (Pb, Cu)(Sr, La)2CuO5-δ combines the doping scheme used in these two compounds, in that both the TI and Sr sites are doped. The starting stoichiometry, the refined scale factors for the impurity phases and the refined site occupancies for oxygen suggests that the stoichiometry (relative to Cu) of the superconducting phase is Pb0.60Cu0.40Sr1.08La0.92Cu04.96. Calculation of the average hole concentration in these compounds from charge summation is difficult with these compounds because the TI/PbO layers provide polarizable charge reservoirs that can participate in substantial covalent bonding and because of the probable mixed-valent nature of Tl and Pb. Nevertheless, bond valence sums calculated for the Cu ions in the CuO2 layers for the three 1201 cuprates do provide a correlation with the values of Tc or the absence of superconductivity. © 1991