Orthorhombic-tetragonal phase relation of YBa2Cu3O7-δ has been determined by applying standard statistical thermodynamics. The resultant phase relation can explain the difference between the boundary value δ determined in situ by increasing temperature at a constant oxygen pressure and that determined at room temperature for quenched samples. It is shown that the orthorhombic-tetragonal phase boundry at room temperature extends to much higher δ value, i.e. 0.8, than hitherto expected. The sample heated to tetragonal region can transform to orthorhombic during the quenching process if δ is smaller than 0.8. From the resistivity data of the quenched and annealed samples, we imply that the role of Cu-O one-dimensional chain on the superconductivity is minor.
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