The orthorhombic and tetragonal phases of Y sub 1 Ba sub 2 Cu sub 3 O sub 9-y

A TEM study is presented of the superconductor Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 9-y}, in two distinct states. The orthorhombic state is found to be Pmmm, in contrast to a recent determination of Pmm2. This state is heavily twinned on the (110) plane; dark-field studies on arrays of twins indicate the almost total lack of strain associated with these defects. Intense illumination with the electron beam produces an irreversible phase transformation to a new tetragonal phase P4/mmm. Experiments under different illumination conditions lead us to associate this transformation with beam heating rather than ionization or knock-on damage. Loss of ordering in the oxygen vacancies is likely to be responsible, although there may also be some change in oxygen stoichiometry. This new tetragonal disordered form seems likely to offer a very useful route to understanding the nature of the superconducting transition in these materials, and the role played by oxygen vacancies in the structure.