Folded acoustic phonons and soft mode dynamics in BaTiO 3 / SrTiO 3 superlattices

A series of BaTiO 3 /SrTiO 3 superlattices with stacking periodicity of 6-10 unit cells were fabricated on (001)MgO substrates using pulsed-laser-deposition technique. Polarized Raman spectra revealed BaTiO 3 -like optical vibrations and confirmed tetragonal symmetry of the superlattices with the c axis being normal to the layers. Due to in-plane compressive stresses generated by the lattice mismatch of the constituting layers, the E(ITO) soft mode was underdamped and markedly shifted to higher frequencies with respect to its analogs in the bulk. The existence of perfect superlattice structure was evident from the observation of the folded acoustic phonons in Raman spectra. The frequencies of the folded phonons matched well with the calculated transverse-acoustic branches of the constituent materials. No phase transitions were revealed in the superlattices below room temperature, while phase transition to the paraelectric state is shifted up to 650-700 K and it is diffused.