Structure and Properties of Tl2Nb2O6+x Phases with the Pyrochlore Structure

We have observed a superstructure derived from the compound Tl0.75Sr1.8Ba0.2CuOy. This compound arises from the "collapse" of the parent oxynitrate compound TlSr2Ba2Cu2O7 (NO3) under controlled thermal decomposition. A combined X-ray and neutron powder diffraction study of the title compound indicates that the superstructure possesses the Pmmm space group, with unit cell parameters (derived from neutron diffraction data) of a=3.7362(3) ?, b=11.5449(9) ?, and c=9.1494(8) ?. This superstructure takes the form of a three-fold repeat along the crystal b-axis produced through local distortions of the thallium and barium/strontium positions. This compound is nonsuperconducting due to the presence of a large number of vacancies in the electronically active copper-oxygen planes