RELAXOR-LIKE BEHAVIOR OF PYROCHLORES

Dielectric dispersion and Raman scattering studies of Cd2Nb2O7 pyrochl ore single crystals in the ferroelectric phase (T < T-c = 196 K < T-s = 205 K) are presented. Below T-c, the behaviour of normalized epsilon ' (T) and epsilon '' (T) With decreasing frequency is typical of that for relaxors and gives evidence for an existence of disordered polar m icroregions. The thermal hysteresis near the second-order phase transi tions at T-s and T-c is due to 'overheated' ferroelectric state. The s low relaxation processes developing on a long-time scale below T-c are ascribed to quasi-nonergodicity of the system long before a glassy st ate appears ( below T-g approximate to 18 K). The intensity of the Ram an-active hard modes arising below T-c changes on cooling towards T-g and on thermal scanning within the ferroelectric phase. These facts im ply that the modes are involved in the relaxation processes caused by disordering the system. The relaxor-like behaviour of the system at T- g < T < T-c is ascribed to a dynamical disorder in alignement of Cd-O( 7th) dipoles due to fast reorientational motions of Cd ions over the s ix equivalent off-center positions and to appearance of the short-rang e orientational order in microregions below T-c. The change in intensi ty of the Raman-active hard modes of Cd2Ta2O7 in the same temperature region as for Cd2Nb2O7 does not exclude an existence of slow relaxatio n processes and disordered state in this pyrochlore as well.