Relaxation processes in the ferroelectric phase of Cd₂Nb₂O₇ far below T_curie

Abstract

Dielectric permittivity (250 Hz-1 MHz) and Raman scattering (10–800 cm⁻¹) studies of Cd₂Nb₂O₇ at temperatures far below T_c = 196 K are performed. The low-frequency dielectric dispersion (250 Hz-10 kHz) which dominates below T_dis < 150 K for light-yellow crystals and below T_dis < 170 K for colourless and red-orange crystals is characterized by the activation energy E_ac≈1.1÷1.6 eV and by the relaxation time τ_o∼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 T_c. 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 T_g (≈ 18 K). The relaxor-like behaviour of the system at T_g < T < T_c 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 ≈ 10T_g. 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 T_g reveals that these modes are involved in the relaxational processes of the system.