Correlation between ferroelectricity and electronic density of states near the Fermi level in BaTiO 3

The electronic density of states in BaTiO 3 is studied by means of X‐ray absorption near‐edge structure in a vicinity of a Ti K edge and Ba L edges. A 4pπ band of the Ti ion and a 5dπ band of the Ba ion having a high‐density unoccupied state occupy a level just above the Fermi level ( E F ). The pre‐edge peaks give that, in addition to an O 2− 2p valence band, the electronic density of states below E F are composed of negative‐charge‐transfer bands 3d 1 , 3d 1 $ \underline {\rm L} $ and 3d 2 $ \underline {\rm L} $ 2 of Ti ions and 4f 1 , 4f 1 $ \underline {\rm L} $ and 4f 2 $ \underline {\rm L} $ 2 of Ba ions. The 3d 1 , 3d 1 $ \underline {\rm L} $ and 3d 2 $ \underline {\rm L} $ 2 bands of Ti ions gives that a mixture of the different oxidation states Ti 2+ and Ti 3+ is present in addition to Ti 4+ . The 4f 1 , 4f 1 $ \underline {\rm L} $ and 4f 2 $ \underline {\rm L} $ 2 bands are induced by a hybridization between Ba 2+ 4d and O 2– 2p orbitals via an electronic configuration interaction between the 4f and 4d orbitals of the Ba ion. The disappearance of the 4f 1 , 4f 1 $ \underline {\rm L} $ and 4f 2 $ \underline {\rm L} $ 2 bands in the ferroelectric phase suggests that the collective displacement of Ba ions from ideal ionic sites of the cubic perovskite‐type structure reduces the hybridization between the Ba and O ions. This gives that the effective charge on the O ions is increased and induces a large dipole moment between the O ions and the Ti ions, which are at an off‐center position of TiO 6 octahedra in the ferroelectric phase. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)