A time-temperature-transformation diagram for the orthorhombic to tetragonal phase transition in YBa2Cu3O7−∂

Abstract
An isothermal transformation curve marking the orthorhombic (O) to tetragonal (T) phase transition boundary has been established over a wide temperature range. There exists within the time‐temperature‐transformation diagram two distinct and separate regions containing superconducting samples with narrow 90‐ and 60‐K transition temperatures, respectively. An oxygen diffusion coefficient at 300 °C in these materials is estimated based on isothermal data. The kinetic process of the TO transformation is studied. It is found, at all temperatures investigated, that extended annealing reduces the normal‐state conductivity of the material and that the best samples are produced for intermediate annealing times.