GRAIN-BOUNDARY MODELING IN HIGH CRITICAL-TEMPERATURE SUPERCONDUCTORS

Geometrical modeling and minimization of energy of grain boundaries in high critical temperature Y-Ba-Cu-O superconducting oxides is used to determine the atomic structure and its influence on the superconducting properties. It is possible to form grain boundaries with four types of atomic planes with different atomic arrangements in the layered structure, thus giving rise to four independent configurations. The structure of the primary coincidence symmetric tilt boundaries containing either a and b or c and a lattice parameters within the grains is illustrated for several misorientations. The modeling is used to determine the fraction of superconducting planes which are continuous across the grain boundary and the excess charge associated with the boundary. The modeling is also applied to secondary coincidence symmetric tilt boundaries and asymmetric tilt boundaries to delineate important features associated with these configurations. The fraction of superconducting planes at the boundaries, which determine their current carrying capacity, is estimated. The relative occupancy of the copper and the oxygen atoms in the coincidence site unit cells is evaluated for each misorientation.