Infrared spectroscopy of an epitaxial BaTiO₃/SrTiO₃ superlattice grown on a (110) SmScO₃ substrate

Polarized infrared reflectance spectra of a BaTiO₃/SrTiO₃ superlattice deposited on the (110) cut of SmScO₃ substrate and bare substrate have been studied in the broad temperature range from 10 to 650 K. Strong infrared anisotropy of the SmScO₃ substrate was observed and explained as a projection of B_1u and B_2u + B_3u phonons into the (110) plane. In the polarization parallel to the c-axis below 200 cm⁻¹, an anomaly in the temperature dependence of the spectra was observed. Fitting the superlattice and substrate spectra, the parameters of optical phonons and their temperature dependence were determined. The superlattice phonon frequencies show only classical behavior, it means frequency hardening on cooling. Anisotropy was also found in the superlattice phonon frequencies and their contributions to static permittivity.