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Atomistic study of defects in YBa2Cu3O7.

The orthorhombic structure of YBa{sub 2}Cu{sub 3}O{sub 7} has been described in terms of the shell model and associated two- and three-body short-range interaction potentials. The empirical potentials are determined by fitting the structure and experimental elastic-constant values. These potentials are used to compute point-defect energies and small-polaron energies. An oxidation reaction of Cu{sup +} and lattice oxygen ions leading to O{sup {minus}} is shown to be probable. Migration energies of oxygen ions by a vacancy mechanism near the Cu-O plane have a computed activation energy on the order of 0.3 eV. Phonon frequencies are computed from the potentials and shown to agree well with experimental Raman data. Bipolarons in the crystal are examined, and the species O{sub 2}{sup 2{minus}} is shown to be rather stable.