A study of the tetragonal-to-orthorhombic transition of YBa/sub 2/Cu/sub 3/O/sub x/ by dilatometry

An YBa/sub 2/Cu/sub 3/O/sub x/ bar specimen was annealed at 9300C followed by water quenching under protection of a quartz sheath, then annealed at a series of temperatures between 3000 - 5000C. Isothermal dilation curves were measured at each annealing temperature. An Avrami type plot of these data gave rise to an activation energy H = 1.2 eV for the tetragonal-to-orthorhombic reaction. It is suggested that this is the activation enthalpy of oxygen diffusion in the tetragonal phase which remains continuous and therefore rate controlling throughout the whole course of transition. Based upon this transition process, the authors expect a fully annealed specimen to be composed of numerous superconducting domains separated by a continuous film of semiconducting tetragonal phase, one of two molecular layers thick. At the points of contact, this film is highly strained by the impinging domains, rendering the material superconductive by S/N/S junctions