The complex dielectric permittivity of PbTiO${}_3$ ceramics in the THz frequency range has been investigated theoretically and by a far-infrared reflectance technique. Besides the well-known polar modes of bulk PbTiO${}_3$, the experiment reveals several additional modes identified as geometrical resonances (i.e., extraneous hybrid excitations created by inhomogeneous depolarization fields). A comparison of the experiment and model calculations suggests that the strong geometrical modes located near 300 and 500 cm${}^{-1}$ are associated with the presence of 90${}^{\circ }$ ferroelectric walls.

• Received 11 June 2011

DOI:https://doi.org/10.1103/PhysRevB.84.092104