Models of electronic defects and self-trapped excitons in Li2O

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The semi-empirical INDO method and the embedded-molecular-cluster model have been applied to calculate the atomic structures and electronic transition energies of the F+, F and F2 centres in Li2O and to simulate the self-trapping process of a triplet exciton. Using the parameters of the Slater-type floating orbitals adjusted to the experimental transition energy (4.13 eV) and hyperfine splitting parameters of the F+ centre, the transition energies of the F and F2 centres were obtained to be 4.82 eV and 3.31 eV, respectively. The lattice relaxation accompanied by formation of a triplet exciton and by two neighbouring triplet excitons are also simulated. The results indicate that a triplet exciton is self-trapped by a shift of an oxygen ion by 0.34 AA along a (100) direction, while defect formation as a consequence of the relaxation of a triplet exciton is very improbable. The authors found that two excitons at the nearest neighbour relax into a more stable relaxed state including an O2- bond, and further to a vacancy-interstitial pair. The results of the simulation were compared with experimental observation of the effects of neutron irradiation and of heavy-ion irradiation of Li2O.