ROLE OF ELASTICITY ASSOCIATED WITH OXYGEN ORDERING IN YBA2CU3OZ

A Monte Carlo technique incorporating elasticity is applied to simulat e oxygen-vacancy ordering and concomitant elastic distortion within th e basal plane of YBa2Cu3Oz. The simulation model employs an extension of an anisotropic Ising model to account for small displacements of co pper, oxygen, and vacancy positions within the basal plane so that the simultaneous evolution of atomic positions and occupancies can be exa mined. Simulations performed with this model are first compared to pre vious studies of oxygen-vacancy ordering under a static lattice approx imation, and then used to investigate the contribution of elastic stra in 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 occurrin g in this system. The influence of elasticity on microstructural evolu tion is examined via simulations of rapid quenching through the tetrag onal-to-orthorhombic transition or of deformation of the orthorhombic phase to induce strain, both followed by annealing. Results are presen ted as lattice configuration photographs, fast Fourier transform inten sity distributions, and strain maps. The formation and evolution of th ese microstructures are rationalized in terms of the accommodation of strain energy accumulated during the course of the simulated thermomec hanical treatments. The formation of the root 2 alpha(0) X root 2 alph a(0), structure is found to result from strain dissipation via a softe ned phonon mode with wave vector q=[1/2 1/2 0].