khoshnevisan - University of Kashan, Kashan, Iran
Ross - Institute for Materials Research, University of Salford, UK

In this study of YBa2Cu;06+, at temperatures 200 &300°C, as the value of x ( the oxygen content in the basal plane) increased under nearly thermodynamical equilibrium condition, the structure changed from the well- known tetragonal (T) to a mixed-phase (T. 011) region (01-tl~orhombicIpIh ase is a superstructure). This mixedphase region suggests a first order TI011 phase transition with the expected linear change in the volume fiaction of each phase (phase lever rule). The existence of a stable whole 011 phase region around x- 0.7 at 300°C. which is indicated b) extrapolation, iniplies a substantial correction 011 the YBCO's structi~ral phase diagram. From the full analyses of the data. the possibility of the formation of another superstructure along the c direction can not be ruled out.