A combined X-ray and neutron-powder diffraction study of a superstructure derived from Tl0.75Sr1.8Ba0.2CuOy

摘要

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) Angstrom, b = 11.5449(9) Angstrom, and c = 9.1494(8) Angstrom. 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, (C) 2000 Academic Press. [References: 10]