By means of X-ray diffraction investigationsand electric resistivity measurements the tempera-ture dependence of phase structure and supercon-ductivity in YBa;Cu;O;over the temperaturerange of 20℃-950℃ have been studied.The latticeparameters a,b and c as well as conductivityas a function of annealing temperatures canbe roughly divided into three zones which aresuperconductive orthorhombic structure 1（a<bc/3）at 20℃-500℃,normal conductive orthorhombicstructure 2（a<b<c/3）from 550℃ to about 750℃and semiconductive-like tetragonal structure（a=b<c/3）from about 800℃ to 950℃.However,the boundaries of the three zones is not muchdistinctive because there is a continuous transi-tion from the orthorhombic to tetragonal struc-ture.Furthermore,the processes of orthorhom-bic-tetragonal transition in the range of350℃-950℃ are not only continuous but alsoreversible.The isothermal transition oforthorhombic 2 to orthorhombic 1 occurs at350℃-500℃,while the maximum rate of transitionand high T;superconductivity can be obtainedat about 460℃.This process of isothermaltransition depends on the ordering of oxygenatoms or vacancies and the thermodynamicequilibrium to allow the oxygen diffusion.
