RELAXATION PROCESSES IN THE FERROELECTRIC PHASE OF CD2NB2O7 FAR BELOWT-CURIE

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 pe rformed. the low-frequency dielectric dispersion (250 Hz-10 kHz) which dominates below T-dis < 150 K for light-yellow crystals and below T-d is < 170 K for colourless and red-orange crystals is characterized by the activation energy E-ac approximate to 1.1 divided by 1.6 eV and by the relaxation time tau o similar to 10(-8)-10(-9) s, typical for rel axors. The temperature and frequency dependences of epsilon give evid ence for an existence of the short-range polar regions (micro- or nano scale) below T-c. an anomalous temperature-dependent dispersion parame ter h(T) implies that cooperative effects and transformation of the di sordered microregions into the long-range clusters are developed in th e system on cooling towards T-g (approximate to 18 K). the relaxor-lik e behaviour of the system at T-g < T < T-c is ascribed to a disorder i n distribution of the Cd ions reoriented at a fast rate >10(7) Hz over the six equivalent off-center sites. The Raman scattering of the hard mode at 214 cm(-1) arises below 180 K approximate to 10T(g). Evolutio n 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.