Grain-boundary constraint and oxygen deficiency in YBa₂Cu₃O_7−δ: Application of the coincidence site lattice model to a non-cubic system

Local lattice parameters and oxygen (hole) densities in the vicinity of large-angle grain boundaries in bulk YBa₂Cu₃O_7−δ were measured simultaneously using convergent-beay electron diffraction and electron-energy-loss spectroscopy with a less than 20 Å field emission probe respectively. The oxygen (hole) contents were quantified by Gaussian fitting to the observed oxygen K-edge absorption spectra and comparing them with results for a set of standard samples with different oxygen contents. The local lattice parameters were determined by matching the simulated high-order Laue zone patterns with experimental patterns by a least-squares algorithm. We found that large-angle grain boundaries in YBa₂Cu₃O_7−δ 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 distortion extends less than 20 Å, while the deficiency can extend more than 100 Å. Comparison of the c/a ratio of the lattice parameters in the boundary regions with those in the interior of the adjacent grains suggests that, to reduce boundary lattice mismatch or to form a coincidence boundary in YBa₂Cu₃O_7−δ the required grain-boundary constraint can be realized by adjusting the oxygen content at the boundary. Our observations convincingly demonstrate the validity of the constrained coincidence site lattice model for studying grain boundaries in non-cubic YBa₂Cu₃O_7−δ.