The electronic band structures of SrBi 2Nb 2O 9 (SBN) in both the ferroelectric and paraelectric phases were calculated using first principles method. The paraelectric phase is of an indirect band gap of 1 57eV ; whereas the ferroelectric phase is of a direct band gap of 2 23eV. The valence band maxima of both the paraelectric and ferroelectric phases are mainly contributed from O2p states; while the conduction band minima are from Nb 4d and Bi 6p states. When transforming from the paraelectric to the...