Point defects of La2CuO4-based ceramics .1. Oxygen interstitials

Theoretical and experimental studies are reported on oxygen excess defect structure of lanthanum cuprate, La2CuO4+delta. The local density functional formalism is applied to theoretical analysis, The self-consistent discrete variational method has been used to find energy levels, densities of states, charge transfer, wavefunctions, and potentials for a fragment consisting of N (30-44) atoms embedded in the infinite crystal, Calculations have been performed on three interstitial oxygen models, with and without local relaxations, These models include sites proposed on the basis of neutron diffraction and interstitial dimers [O-2](q). The dimer charge q is found to be consistent with that of normal oxygens, and not -1 as proposed in ''superoxide'' models, Electrostatic energies are used to estimate the relative stabilities of the proposed interstitial structures, The experimental Seebeck coefficient also shows that the charge of interstitial oxygen is -2. Effective atomic configurations for the ideal lattice are compared with those in the vicinity of the defects, and defect-related spectroscopic consequences are examined.