Polarization switching along a substrate in thin Bi$_{4}$Ti$_{3}$O$_{12}$ films under different deformation stresses

Single-crystal Bi$_{4}$Ti$_{3}$O$_{12}$ films deposited on a 4-nm-thick Ba$_{0.4}$Sr$_{0.6}$TiO$_{3}$ sublayer covering an MgO(001) substrate have been investigated. It has been found that the unit cells of Bi4Ti3O12 films in the resulting heterostructures are turned by 45$^\circ$ relative to the cells of the MgO substrate in the interface plane. In addition, the unit cells of the films are strained, the amount of strain depending on the Bi$_{4}$Ti$_{3}$O$_{12}$ film thickness. When the films become about 40 nm thick, strain changes sign. It has been shown that reversible spontaneous polarization in Bi$_{4}$Ti$_{3}$O$_{12}$ films with a 180$^\circ$ domain structure in the interface plane arises at a thickness of 10 nm and grows with thickness to 54 $\mu$C/cm$^2$. The anisotropy of the film’s properties in the interface plane and the influence of unit cell distortion on the properties of the heterostructures have been confirmed by studying the dielectric performance of the films.