The crystal structures of Bi_(4)Ti_(3)O_(12) and Bi_(3.25)La_(0.75)Ti_(3)O_(12) 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 Bi_(3.25)La_(0.75)Ti_(3)O_(12), 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 Bi_(4)Ti_(3)O_(12) and Bi_(3.25)La_(0.75)Ti_(3)O_(12) systems.
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