DIELECTRIC RESPONSE OF MICROCOMPOSITE FERROELECTRICS

The dielectric response in two-phase composites of ferroelectric and d ielectric materials was studied. Different effective medium theories ( isolated particles, Maxwell-Garnet theory, the effective medium approx imation and the Bergman representation of the effective dielectric fun ction) were used together with the theory of absorption in finite ferr oelectric crystals in the quasistatic approximation to describe the re sponse of the composite. One of the main results concerns the existenc e of modes due to geometric resonances which among other effects can l ead to new low-frequency absorption peaks near the percolation thresho ld of the ferroelectric material. These peaks appear always higher tha n the transverse polar mode frequencies of the bulk components but bel ow their corresponding longitudinal mode frequencies. The soft mode wi th renormalized mode strength but unchanged frequency exists only in t he composite with percolated ferroelectric clusters. When the ferroele ctric clusters are only finite the soft mode does not soften completel y.