Chain-plane oxygen rearrangement in YBa2Cu3Oz at room temperature. Thermodynamic equilibrium may be obtained in a reasonable period of time

In a previous study, the boundary between the orthorhombic (O) and tetragonal (T) phases of YBa2Cu3Ox has been determined down to about 100degC by measurement of (dLnR/dT)z(T), which suggests that the chain-plane oxygen rearrangement corresponding to the O-T transition occurs with very favorable kinetics. By determining constant-T R relaxation curves at different temperatures, including room temperature and compositions z, it has been possible to demonstrate that thermodynamic equilibrium may be obtained at room temperature by annealing during a period of about one week. Moreover, samples of the same composition but prepared by quenching and by slow cooling at constant composition z, exhibit after room temperature annealing, the same final state as determined by their critical temperature, Tc. During this annealing, the conductivity, σ, increases along with Tc, the Tc and σ variations being proportional, which suggests that the chain-plane oxygen rearrangement produces an increase of the positive hole concentration in the CuO2 planes and that Tc is proportional to this concentration. (orig.)