Lattice dynamics and broad-band dielectric properties of multiferroic Pb(Fe1/2Nb1/2)O3 ceramics.

Complex dielectric properties of PbFe_1/2Nb_1/2)O₃ ceramics were investigated in a broad frequency range from 100 Hz up to 90 THz. A broad dielectric anomaly was observed near the temperature of the ferroelectric phase transition (T_C1=376 K). Below 1MHz, the anomaly is strongly influenced by conductivity of the sample, but higher frequency data taken up to 81 MHz reveal a broad and frequency independent peak at T_C1 typical for a diffuse ferroelectric phase transition. Surprisingly, dielectric permittivity measured at 37 GHz exhibits a peak shifted by 25K above T_C1, which indicates polar nanoregions with dynamics in microwave frequency region. A dielectric relaxation, which appears in THz region below 700 K, slows down towards T_C1 and again hardens below T_C2=356 K. This central mode drives both phase transitions, so they belong to order-disorder type, although the polar phonons exhibit anomalies near both phase transitions. In the paraelectric phase, infrared reflectivity spectra correspond to local Fm3m structure due to short-range chemical ordering of Fe and Nb cations on the B perovskite sites. Moreover, each polar phonon is split due to two different cations on the B sites. Recently, Manley et al. [Nat. Commun. 5, 3683 (2014)] proposed a new mechanism of creation of polar nanoregions in relaxor ferroelectrics. They argued, based on their inelastic neutron scattering studies of PMN-PT, that the TO1 phonon is split and interaction of both components gives rise to so called Anderson phonon localization, which can produce regions of trapped standing waves and these waves induce polar nanoregions in relaxors. We cannot exclude or confirm this mechanism, but we show that the splitting of polar phonons is a common feature for all complex perovskites with relaxor ferroelectric behavior and it can be also observed in canonical ferroelectric BaTiO₃, where the soft mode is split in paraelectric phase due to a strong lattice anharmonicity. [ABSTRACT FROM AUTHOR]