Lattice structure and dynamics of two-layer heterostructures of barium–strontium titanate and layered bismuth titanate of various thicknesses on a magnesium oxide substrate

The lattice structure and dynamics of single-crystal Bi$_{4}$Ti$_{3}$O$_{12}$ with a thickness from 4 to 430 nm on a (001)MgO substrate with a previously deposited Ba0.4Sr0.6TiO3 sublayer (4 nm) have been studied. The two-layer structures were prepared by radio-frequency sputtering of ceramic targets of the corresponding compositions. The X-ray diffraction studies performed at room temperature have shown that, in this heterostructure, axis c of the Bi$_{4}$Ti$_{3}$O$_{12}$ unit cell is perpendicular to the substrate, and direction [100] has an angle of $\pm$ 45$^\circ$ to the [100]MgO direction. At the thicknesses of Bi$_{4}$Ti$_{3}$O$_{12}$ to $\sim$40 nm, the film unit cell is compressed in the direction of a normal to the substrate plane and extended in the conjugate plane, and the sign of the deformation is changed at large thicknesses. It is found that the phonon mode frequency in the Bi$_{4}$Ti$_{3}$O$_{12}$ film shift and additional peaks appear in the Raman spectra, which demonstrates an increase in the degree of monoclinic distortion of the film crystal structure as compared to the crystal structure.