ATOMISTIC SIMULATION STUDIES OF TRAPPED HOLE BIPOLARONS IN BATIO3

Classical shell-model- and embedded-cluster-type calculations are employed in order to supply theoretical arguments in favour of hole bipolarons in BaTiO3 which have recently been speculated to exist in this photorefractive material. Our investigations concern the geometrical structure of hole bipolarons trapped at acceptor defects, their spin state and hole ionization energies. In particular the embedded-cluster modelling studies, which explicitly include the local electronic defect structure, suggest the importance of lattice relaxation and electron correlation terms in order to stabilize diamagnetic O-2(2-) molecules (bipolarons) in BaTiO3. Our simulations show that hole bipolarons are predominantly bound at Ti-site acceptor defects. A trapping of bipolarons at Ba-site accepters is in most Cases unfavourable. Finally, by extrapolating our present results to the high-T-c superconducting oxides we qualitatively discuss the possible role of small hole (peroxy) bipolarons in these materials.