X-Ray Single Crystal Structure Analysis of Tetragonal Ba2YCu3-xO7-y, a High Temperature Form of the Superconductor

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Abstract
Crystal data: tetragonal, space group P4/mmm, a=3.8640(4), c=11.755(3) Å, final R factor=0.036 for 161 independent observed reflections. Cu1 and two O1 form linear O-Cu-O dumbell (Cu-O1 1.808 Å) with additional square planer O2 group (site occupancy=0.2) around Cu1. Cu2 is in square pyramidal coordination of oxygen composed of square-planer O3 group (Cu-O3 1.944 Å) and an apical O1 (Cu-O1 2.434 Å). The Y and Ba are in ordered arrangement along the c axis. This tetragonal phase is a high temperature form of the orthorhombic phase and the transition is related with ordering of the oxygen vacancy. Presence of strong metal-metal interactions were presumed because there exist Ba-Ba, Ba-Y and Ba-Cu distances significantly shorter than sum of their corresponding metallic radii.