Phase transitions in two- and three-component perovskite superlattices

The temperature behavior of polarization Raman spectra of two-layer BaTiO$_3$/Ba$_{0.5}$Sr$_{0.5}$TiO$_3$/SrTiO$_3$ and three-layer BaTiO$_3$/Ba$_{0.5}$Sr$_{0.5}$TiO$_3$/SrTiO$_3$ superlattices prepared by pulsed laser deposition on (001)MgO substrates has been studied. It has been found that, at room temperature, the frequency of soft mode $E$(TO) in the three-layer superlattice is significantly higher (94 cm$^{-1}$) than that in the two-layer superlattice (70 cm$^{-1}$), which demonstrates an increase in two-dimensional stresses in epitaxial layers of the superlattice. An analysis of the temperature dependence of the integrated intensity of soft mode $E$(TO) suggests that the BaTiO$_3$/Ba$_{0.5}$Sr$_{0.5}$TiO$_3$/SrTiO$_3$ superlattice undergoes a transition from the ferroelectric to paraelectric phase at $\sim$ 540 K, and the phase transition temperature of the BaTiO$_3$/Ba$_{0.5}$Sr$_{0.5}$TiO$_3$/SrTiO$_3$ superlattice increases to $\sim$ 610 K because of the deformation of the epitaxial layers.