ROLE OF ELASTICITY ASSOCIATED WITH OXYGEN ORDERING IN YBA2CU3OZ

A Monte Carlo technique incorporating elasticity is applied to simulate oxygen-vacancy ordering and concomitant elastic distortion within the basal plane of YBa2Cu3Oz. The simulation model employs an extension of an anisotropic Ising model to account for small displacements of copper, oxygen, and vacancy positions within the basal plane so that the simultaneous evolution of atomic positions and occupancies can be examined. Simulations performed with this model are first compared to previous studies of oxygen-vacancy ordering under a static lattice approximation, and then used to investigate the contribution of elastic strain to the formation of experimentally observed root 2 superstructures, deformation tweed, twinning, nucleation and growth of ordered domains, and the nature of the tetragonal-to-orthorhombic transition occurring in this system. The influence of elasticity on microstructural evolution is examined via simulations of rapid quenching through the tetragonal-to-orthorhombic transition or of deformation of the orthorhombic phase to induce strain, both followed by annealing. Results are presented as lattice configuration photographs, fast Fourier transform intensity distributions, and strain maps. The formation and evolution of these microstructures are rationalized in terms of the accommodation of strain energy accumulated during the course of the simulated thermomechanical treatments. The formation of the root 2 alpha(0) X root 2 alpha(0), structure is found to result from strain dissipation via a softened phonon mode with wave vector q=[1/2 1/2 0].