Direct HVEM observations of transitions in YBa2Cu3O7−x in the presence of a controlled oxygen atmosphere

Abstract The tetragonal to orthorhombic (t→o) phase transition in YBa 2 Cu 3 O y thin foils has been induced by exposure to oxygen pressures between 40 and 150 Torr at 500 to 600°C, and directly observed within the environmental cell of a high voltage transmission electron microscope, using a heating stage. The value of y increased from about 6.2 to about 6.75, and transformation twins appeared, except in very thin areas of the samples. The structure transformed back to tetragonal when oxygen pressure was reduced to 10 Torr or less. The t→o transition is controlled by a very slow oxygen adsorption rate, while the o→t transition is faster, and is controlled by oxygen diffusion or desorption.