Electron-density distributions of paraelectric and ferroelectric ${\mathrm{SrBi}}_2{\mathrm{Nb}}_2{\mathrm{O}}_9$ 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 ${\mathrm{Bi}}_2{\mathrm{O}}_2$ layer. The hybridization between $\mathrm{Nb}\mathrm{}\left(4d\right)\mathrm{}$ and $\mathrm{O}\mathrm{}\left(2p\right)\mathrm{}$ 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 $\mathrm{Sr}\mathrm{}\left(5s\right)\mathrm{}$ with $\mathrm{O}\mathrm{}\left(2p\right)\mathrm{}$ 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 ${\mathrm{PbTiO}}_3$ perovskite.

• Received 19 June 2002

DOI:https://doi.org/10.1103/PhysRevB.66.144110