The crystal structures of Bi4Ti3O12 and Bi3.25La0.75Ti3O12 were refined by neutron powder diffraction. Large structural distortions were revealed, and ferroelectric polarizations along the a and c axes were calculated from the displacements of the constituent ions. In Bi3.25La0.75Ti3O12, La atoms substitute for Bi atoms in a perovskite-type unit only, and the substitution causes less distortion of the structure, resulting in smaller spontaneous polarization and lower ferroelectric Curie temperature. Electronic-structure calculations revealed that covalent interaction, which originates from the strong hybridization between Ti 3d and O 2p orbitals, plays an important role in the structural distortion and ferroelectricity of the materials. Changes in ceramic-sample density with sintering temperature give information concerning device fabrication temperature; that is, substituting La for Bi atoms appears to “increase” the synthesis temperature of the Bi4Ti3O12 and Bi3.25La0.75Ti3O12 systems. © 2001 American Institute of Physics.