Theoretical modeling and experimental characterization of planar defects in Y sub 2 Ba sub 4 Cu sub 6+x O sub 14+x

Crystallographic defects and phase transformations in the system Y{sub 2}Ba{sub 4}Cu{sub 6 + x}O{sub 14 + x}(0{le}{times}{le}4) are investigated by high resolution transmission electron microscopy (TEM) and static lattice, three dimensional Monte Carlo computer simulations. High resolution images of partially transformed (x = 2 to x = 1) material reveal a prevalence of CuO planar defects (stacking faults) associated with the transformation and an absence of disturbance to the perovskite Ba-Y-Ba blocks. An atomic mechanism involving the intercalculation and removal of extra CuO planes by partial dislocation climb, and requiring only a-b plane diffusion, is developed for the formation of such planar defects during changes in the layered YBaCuO crystal structure. Monte Carlo simulations based on the proposed transformation mechanism accurately reproduce the observed defects and known equilibrium structures. 24 refs., 4 figs