Oxygen deficiency in Ba₂YCu₃O_7-δ: The need for a microscopic description

The physical properties, such as crystal structure, resistivity, T_c and normal state susceptibility have been studied in Ba₂YCu₃O_7-δ as a function of oxygen deficiency δ. Oxygen was removed in a controlled way at relatively low temperatures employing a Zr-gettering technique. Not only do we find a second superconducting phase with T_c∼60 K and with new (short-range) crystallographic periodicity along the b-direction, but we also find that all physical properties depend on the manner in which oxygen is removed. Variations of physical properties among samples with the same over-all oxygen content can be consistently associated with the occupation of the various O-sites in the structure.