Oxygen ordering and the orthorhombic-to-tetragonal phase transition in YBa_{2}Cu_{3}O_{7-x}

In situ neutron powder diffraction measurements show that the orthorhombic-to-tetragonal phase transition in YBa2Cu307 —,which occurs near 700 C in a pure oxygen atmosphere, is an order-disorder transition in which the disordering of oxygen atoms into a normally vacant site destroys the one-dimensional Cu-0 chains present in the room-temperature orthorhombic structure. For both structures, the oxygen stoichiometry decreases monotonically with increasing temperature. The transition temperature depends on the oxygen partial pressure and occurs when the stoichiometry is near YBa2Cu3065. The tetragonal structure has a partially occupied, nearly octahedral Cu-0 arrangement, in contrast to the orthorhombic structure which has one-dimensional Cu-0 chains. The observed depression of the superconducting transition temperature in tetragonal YBazCu307 —„, which has been quenched from high temperature, could result either from the disordering of oxygen atoms which destroys the one-dimensional chains or from the absence of Cu + ions.