  First-principles calculations were performed to investigate the ferroelectric
properties of barium titanate and bismuth ferrite, as well as phonon dispersion
of BaTiO3, using density functional theory and density functional perturbation
theory. Results show that the strong hybridization of Ti-O and Bi-O lead to the
corresponding mechanisms for stabilizing the distorted structure. The
spontaneous polarization of 59.4 \mu C/cm2 and 27.6 \mu C/cm2 were calculated
for BiFeO3 and BaTiO3 respectively, using berry phase method within the modern
theory of polarization. The stereochemical activity of Bi-6s long-pair, which
was the driven mechanism for ferroelectricity in BiFeO3, was able to produce
greater polarization than the Ti off-centring displacement in BaTiO3. New
multiferroic perovskite type materials combined with these two ferroelectric
instabilities were predicted to have a better ferromagnetic ordering in
comparison with BiFeO3.
