An in situ powder diffraction study of the structural mixed-phase of YBa2Cu3O6+x at 200 and 300C

In this study, the YBa2Cu3O6+x samples at temperatures 200 and 300°C, as the value of x ( the oxygen content in 0the basal plane)  increased under nearly thermodynamical equilibrium condition, the structure changed from the well- known tetragonal (T) to a mixed-phase (T+OII) region (orthorhombicII phase is a superstructure). This mixed-phase region suggests a first order T/OII phase transition with the expected linear change in the volume fraction of each phase (phase lever rule). The existence of a stable whole OII phase region around x» 0.7 at 300°C, which is indicated by extrapolation, implies a substantial correction on the YBCO’s structural phase diagram. From the full analyses of the data by Reitveld Method, the possibility of the formation of another superstructure along the c direction can not be ruled out (broadening of [00l] reflections which, presumably comes from anti phase boundary effects). By inserting the structure factor of the OII phase, we have been able to study this phase quantitatively.