INFRARED AND RAMAN SPECTROSCOPY OF SOME FERROELECTRIC PEROVSKITE FILMS AND CERAMICS

The problem of reduced dielectric response in thin films compared with single crystals is addressed for SrTiO3 using far infrared and micro-Raman spectroscopy. It is shown that the soft-mode behaviour, which is responsible for the dielectric response, is extremely sensitive to the film quality. In epitaxial films the stress from the substrate, which can easily change the phase diagram (e.g. induce ferroelectricity), plays the most decisive role. In polycrystalline films and ceramics, grain boundaries and possible nano-cracks, treated using the brick-wall model, can explain the observed dielectric behaviour. Soft-mode behaviour in many other perovskite films and ceramics from the BST and PZT systems is briefly discussed. In the case of PbZrO3 ceramics, the effect of nano-cracks on the dynamic dielectric response is studied experimentally and explained by the brick-wall model.