Polarization rotation in Bi4Ti3O12 by isovalent doping at the fluorite sublattice

Bismuth titanate, Bi4Ti3O12 (BiT), is a complex layered ferroelectric material that is composed of three perovskitelike units and one fluoritelike unit stacked alternatively along the transverse direction. The ground-state crystal structure is monoclinic with the spontaneous polarization (similar to 50 mu C/cm(2)) along the plane. BiT typically grows along the c direction in thin-film form, and having the polarization vector aligned with the growth orientation can be beneficial for several potential device applications. It is well known that judicious doping of ferroelectrics is an effective method in adjusting the magnitude and the orientation of the spontaneous polarization. Here, we show using first-principles density-functional theory and a detailed phonon analysis that Bi atoms in the fluoritelike layers have significantly more impact on the magnitude and orientation of the spontaneous polarization vector as compared to the perovskitelike layer. The low-energy hard-phonon modes are characterized by fluoritelike layers experiencing transverse displacements and large changes in Born effective charges on Bi atoms. Thus, the breaking of symmetry caused by doping of Bi sites within the fluoritelike layer leads to the formation of uncancelled permanent dipole moments along the transverse direction. This provides an opportunity for doping the Bi site in the fluoritelike layer. Isovalent dopants P, As, and Sb were studied. P is found to be most effective in the reorientation of the spontaneous polarization. It leads to a threefold enhancement of the out-of-plane component of polarization and to a commensurate rotation of the spontaneous polarization vector by 36.2 degrees towards the transverse direction.