Crystal and electronic structures of ${\mathrm{Bi}}_{\text{4−x}}{\mathrm{La}}_x{\mathrm{Ti}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}}$ ferroelectric materials

The crystal structures of ${\mathrm{Bi}}_{\mathrm{4}}{\mathrm{Ti}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}}$ and ${\mathrm{Bi}}_{\mathrm{3.25}}{\mathrm{La}}_{\mathrm{0.75}}{\mathrm{Ti}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}}$ 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 ${\mathrm{Bi}}_{\mathrm{3.25}}{\mathrm{La}}_{\mathrm{0.75}}{\mathrm{Ti}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}},$ 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 $\text{3d}$ and O $\text{2p}$ 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 ${\mathrm{Bi}}_{\mathrm{4}}{\mathrm{Ti}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}}$ and ${\mathrm{Bi}}_{\mathrm{3.25}}{\mathrm{La}}_{\mathrm{0.75}}{\mathrm{Ti}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}}$ systems.