Dielectric response of microcomposite ferroelectrics

The dielectric response in two-phase composites of ferroelectric and dielectric materials was studied. Different effective medium theories (isolated particles, Maxwell-Garnet theory, the effective medium approximation and the Bergman representation of the effective dielectric function) were used together with the theory of absorption in finite ferroelectric crystals in the quasistatic approximation to describe the response of the composite. One of the main results concerns the existence of modes due to geometric resonances which among other effects can lead to new low-frequency absorption peaks near the percolation threshold of the ferroelectric material. These peaks appear always higher than the transverse polar mode frequencies of the bulk components but below their corresponding longitudinal mode frequencies. The soft mode with renormalized mode strength but unchanged frequency exists only in the composite with percolated ferroelectric clusters. When the ferroelectric clusters are only finite the soft mode does not soften completely.