Soft-mode spectroscopy of BaTiO3 thin films

We present a detailed study of one highly ordered and two polycrystalline BaTiO3 films of different thicknesses by far infrared and micro-Raman spectroscopy in the 10–550 K temperature range. The lowest-frequency phonon mode exhibits rather week and smeared softening in comparison with its single crystal behaviour in all three films. A simultaneous presence of three soft-mode components, continuously changing with the temperature, is assigned to a gradual symmetry transformation within the film volumes. A modification of the soft-mode splitting in the highly ordered film can be explained by a constraint from the substrate. Polycrystalline films exhibit presence of polar modes in the Raman spectra up to 150 K above the single-crystal transition TC.