  High-frequency dielectric studies of Bi_0.85Nd_0.15FeO_3 ceramics performed
betweeen 100 and 900 K reveal hardening of most polar phonons on cooling below
antiferroelectric phase transition, which occurs near 600 K. Moreover, a strong
THz dielectric relaxation is seen in paraelectric phase. Its relaxation
frequency softens on cooling towards Tc ~ 600 K, its dielectric strength
simultaneously decreases and finaly the relaxation disappears from the spectra
below 450 K. Both phonon and dielectric relaxation behavior is responsible for
a decrease in the dielectric permittivity at the antiferroelectric phase
transition. Origin of unusual strong THz dielectric relaxation in paraelectric
phase is discussed. Bi_0.85Nd_0.15FeO_3 structrure lies on the phase boundary
between polar rhombohedral and non-polar orthorhombic phase and owing to this
the polarization rotation and polarization extension can enhance the
piezoelectric response of this system. Similarities and discrepancies with
lead-based piezoelectric perovskites, exhibiting morphotrophic phase boundary
between two ferroelectric phases, are discussed.
