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The structure and copper valence states of the 100K superconductor, Ba₂YCu₃O₇ have recently been determined by neutron powder diffraction between 5K and 300K¹, and at room temperature². This ′oxygen deficient orthorhombic perovskite structure′, now independently confirmed^3,4, is well ordered and stochiometric, and contains one Cu⁺⁺⁺ and two Cu⁺⁺ atoms distributed over two square planar oxygen co-ordinated sites. In the a-b plane CuO₄ squares are linked by their oxygen corners to form infinite sheets, while along the b-axis CuO₄ squares form infinite chains on the second Cu-site. In this paper we show that when Ba₂YCu₃O₇ is heated above room temperature, it progressively loses all oxygen from the O4 sites on the b-axis chains, while the remaining oxygen sites remain fully occupied. Near 700°C it becomes tetragonal⁵, with the P4/mmm Ba₂YCu₃O₆ structure obtained by X-ray diffraction at …