FIR and near-millimetre dielectric response of SrTiO3, BaTiO3 and BST films and ceramics

The dielectric response of Ba1−xSrxTiO3 ceramics and films (x=0, 0.9, 1) was studied with a particular attention to the soft mode behaviour in the far IR and near-mm range and its comparison with the single crystal behaviour. To analyse the response quantitatively, we used commercial Fourier transform IR and monochromatic backward-wave-oscillator spectroscopy in both transmission and reflection modes and a rigorous evaluation. The soft-mode behaviour is extremely sensitive to the sample quality, particularly in films, and correlates with the lower low-frequency response. The most important factors for this behaviour in the polycrystalline films are grain boundaries and porosity and nano-cracks, which often appear in thicker polycrystalline films along some of the grain boundaries. The brick-wall model for grain boundaries and cracks discussed within the generalized effective medium approximation appears to be appropriate for describing the observed phenomena. In quasi-epitaxial films the macroscopic tensile stress, which influences the phase diagram very sensitively and induces ferroelectricity even in the pure SrTiO3, seems to play the most important role. From the soft-mode behaviour and its splitting, it appears that in the studied BaTiO3 films all the phase transitions, as in single crystals, seem to be present, but smeared with phase coexistence regions down to 10 K. In the quasi-epitaxial BST-0.9 film a smeared ferroelectric transition appears near 150 K.