The transformation processes during synthesis have been studied with in situ high temperature X—ray diffraction for YBa/2/Cu/3/O/9-x/oxide.Formation of superconducting phase with high zero—resistance temperature above 90K is developed from a series of transformation. On heating

a transitive cubic phase appears at 880℃ at first. It then transforms into tetragonal until 900℃. As extension of holding time at 960℃

tetragonal structure transforms into orthorhombic gradually. On cooling

a second orthorhombic transition occurs at 600℃. Zero—resistance superconductivity temperature Tc increases from 77.0 K to 91 K as holding time at 960℃ extends’ from 2 hr. to 6 hr. followed by cooling at same condition. It shows that

superconductivity has been affected by two kinds of orthorhombic transition occured at high temperature and during cooling stage respectively