TlBa1.2La0.8CuO5: A superconductor produced by reduction, rather than oxidation, of the CuO2 planes

Doping of La3+ for Ba2+ in TlBa2CuO5, to introduce holes into the CuO2 planes, leads to superconductivity in the material TlBa1.2La0.8CuO5 (with Tc ≈ 52 K). The structure (P4/mmm) has been refined using neutron powder diffraction data collected at 305 K ( a = 3.8479 (3) A and c = 9.0909 (6) A ) and 15 K ( a = 3.8444 (1) A and c = 9.0744 (3) A ). An increase in the Cu-O in-plane bond distance from that in TlBa2CuO5 is consistent with a lowering of the formal oxidation state of copper from its value of 3+. Substantial static site disorder is observed in the Tl-O (3) planes, presumably to allow for better Tl3+-O coordination. Examination of the thermal parameters for the CuO6 octahedra shows that this disorder in turn produces a local tilting of the octahedra and a resultant puckering of the CuO2 planes.