Phase transitions in ferroelectric Pb0.5Sr0.5TiO3 films probed by spectroscopic ellipsometry

Phase transitions occurring in 130-nm-thick films of perovskite-structure ferroelectric Pb0.5Sr0.5TiO3 are experimentally studied by combining spectroscopic ellipsometry and low-frequency dielectric analysis. Polycrystalline and polydomain epitaxial films with relaxed misfit strain and columnar microstructure are investigated. The paraelectric and the ferroelectric states, and the temperatures and widths of the paraelectric-to-ferroelectric phase transitions, are identified from the temperature evolution of refractive index measured in transparency range. The temperatures at which transitions start on cooling are found to be considerably higher than the temperatures of the dielectric peaks. In contrast to the broad dielectric peaks, the transition width of 60 K in the polycrystalline film and that of 20 K in the polydomain epitaxial film are revealed. The discrepancies between optical and dielectric data are explained by the influence of extrinsic factors on the low-frequency response of the thin-film capacitors. It is suggested that fundamental mechanisms of ferroelectric phase transitions in thin films can be revealed by studies of thermo-optical properties.