STRUCTURAL-PROPERTIES OF YBA2CU3O6+X AT ELEVATED-TEMPERATURES IN CONTROLLED OXYGEN ATMOSPHERES

The structural properties of YBa2Cu3O6+x have been determined at elevated temperatures in controlled oxygen partial pressure by in situ neutron powder diffraction. Diffraction data were obtained as a function of the oxygen partial pressure at constant temperatures of 490-degrees-C and 440-degrees-C. These data, taken in thermodynamic equilibrium, provide information that cannot be obtained from metastable oxygen-deficient samples studied at room temperature. For example, the orthorhombic-to-tetragonal phase transition is observed to be continuous, as expected for a second order phase transition. Sequential diffraction measurements following each change in experimental conditions were used to monitor the approach to equilibrium by observing changes in lattice constants. A considerable increase in equilibration time (following a step-like change in oxygen partial pressure) was found as the orthorhombic-to-tetragonal phase transition was approached. No phases other than the high-temperature orthorhombic (ortho-I) and tetragonal phases were observed.