Orbital hybridization and covalency in paraelectric and ferroelectric (formula presented)

Electron-density distributions of paraelectric and ferroelectric (formula presented) 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 (formula presented) layer. The hybridization between (formula presented) and (formula presented) 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 (formula presented) with (formula presented) 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 (formula presented) perovskite.