Orbital hybridization and covalency in paraelectric and ferroelectric SrBi₂Nb₂O₉

Electron-density distributions of paraelectric and ferroelectric SrBi₂Nb₂O₉ were analyzed by means of synchrotron x-ray diffraction. The obtained electron-density distributions well reproduced the atomic arrangements of the material. Bonding electrons due to hybridization of orbitals were clearly seen in the Nb-O bonds in the octahedron and in the Bi-O bonds in the Bi₂O₂ layer. The hybridization between Nb(4d) and O(2p) orbitals was enhanced in the ferroelectric phase, and one of the Nb-O bonds showed a strong covalent character below the ferroelectric Curie temperature. Weak hybridization of the Sr(5s) with O(2p) orbitals appears to play an important role in structural distortion and ferroelectric polarization. The changes in the electron-density distribution through the ferroelectric phase transition are similar to those observed in ferroelectric PbTiO₃ perovskite.