Orthorhombic-II Phase in Ba2YCu3O7-X Crystals

Abstract
The structure in a wide oxygen-deficiency range of Ba2YCu3O7-X crystal (0.15<XT c∼60 K, X∼0.5), whose shorter a-axis is doubled, is considered to be an ordered oxygen-vacancy arrangement caused by removing oxygen atoms from alternating oxygen atoms and their vacancy arrays along the [010] direction on Cu(1)-plane. The mechanism for the ortho-I →the ortho-II →the “incipient” tetragonal phase transformation is discussed in terms of oxygen-deficiency.