Relaxation processes in the ferroelectric phase of Cd₂Nb₂O₇far below T_curie

Abstract Dielectric permittivity (250 Hz-1 MHz) and Raman scattering (10–800 cm−1) studies of Cd2Nb2O7 at temperatures far below Tc = 196 K are performed. The low-frequency dielectric dispersion (250 Hz-10 kHz) which dominates below Tdis < 150 K for light-yellow crystals and below Tdis < 170 K for colourless and red-orange crystals is characterized by the activation energy Eac≈1.1÷1.6 eV and by the relaxation time τo∼10−8 − 10−9 s, typical for relaxors. The temperature and frequency dependences of ε* give evidence for an existence of the short-range polar regions (micro- or nanoscale) below Tc. An anomalous temperature-dependent dispersion parameter h(T) implies that cooperative effects and transformation of the disordered microregions into the long-range clusters are developed in the system on cooling towards Tg (≈ 18 K). The relaxor-like behaviour of the system at Tg < T < Tc is ascribed to a disorder in distribution of the Cd ions reoriented at a fast rate >107 Hz over the six equivalent off-center sites. The Raman scattering of the hard mode at 214 cm−1 arises below 180 K ≈ 10Tg. Evolution of the intensity of this mode and of the modes at 84 cm−1, 272 cm−1 and 615 cm−1 on cooling towards Tg reveals that these modes are involved in the relaxational processes of the system.

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