GRAIN-BOUNDARY CONSTRAINT AND OXYGEN DEFICIENCY IN YBA2CU3O7-DELTA - APPLICATION OF THE COINCIDENCE SITE LATTICE MODEL TO A NONCUBIC SYSTEM

Local lattice parameters and oxygen (hole) densities in the vicinity o f large-angle grain boundaries in bulk YBa2Cu3O7-delta were measured s imultaneously using convergent-beam electron diffraction and electron- energy-loss spectroscopy with a less than 20 angstrom field emission p robe respectively. The oxygen (hole) contents were quantified by Gauss ian fitting to the observed oxygen K-edge absorption spectra and compa ring them with results for a set of standard samples with different ox ygen contents. The local lattice parameters were determined by matchin g the simulated high-order Laue zone patterns with experimental patter ns by a least-squares algorithm. We found that large-angle grain bound aries in YBa2Cu3O7-delta often exhibit volume expansion. The increase of the c-lattice parameter near the boundary core can be attributed to lattice distortion and/or to an oxygen deficiency. The lattice distor tion extends less than 20 angstrom, while the deficiency can extend mo re than 100 angstrom. Comparison of the c/a ratio of the lattice param eters in the boundary regions with those in the interior of the adjace nt grains suggests that, to reduce boundary lattice mismatch or to for m a coincidence boundary in YBa2Cu3O7-delta, the required grain-bounda ry constraint can be realized by adjusting the oxygen content at the b oundary. Our observations convincingly demonstrate the validity of the constrained coincidence site lattice model for studying grain boundar ies in non-cubic YBa2Cu3O7-delta.