ATOMISTIC SIMULATION STUDIES OF TRAPPED HOLE BIPOLARONS IN BATIO3

Classical shell-model- and embedded-cluster-type calculations are empl oyed in order to supply theoretical arguments in favour of hole bipola rons in BaTiO3 which have recently been speculated to exist in this ph otorefractive material. Our investigations concern the geometrical str ucture of hole bipolarons trapped at acceptor defects, their spin stat e and hole ionization energies. In particular the embedded-cluster mod elling studies, which explicitly include the local electronic defect s tructure, suggest the importance of lattice relaxation and electron co rrelation terms in order to stabilize diamagnetic O-2(2-) molecules (b ipolarons) in BaTiO3. Our simulations show that hole bipolarons are pr edominantly bound at Ti-site acceptor defects. A trapping of bipolaron s at Ba-site accepters is in most Cases unfavourable. Finally, by extr apolating our present results to the high-T-c superconducting oxides w e qualitatively discuss the possible role of small hole (peroxy) bipol arons in these materials.