Relaxor-like behaviour of pyrochlores

Dielectric dispersion and Raman scattering studies of Cd2Nb2O7 pyrochlore 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 microregions. The thermal hysteresis near the second-order phase transitions at T-s and T-c is due to 'overheated' ferroelectric state. The slow relaxation processes developing on a long-time scale below T-c are ascribed to quasi-nonergodicity of the system long before a glassy state appears ( below T-g approximate to 18 K). The intensity of the Raman-active hard modes arising below T-c changes on cooling towards T-g and on thermal scanning within the ferroelectric phase. These facts imply 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 six equivalent off-center positions and to appearance of the short-range orientational order in microregions below T-c. The change in intensity of the Raman-active hard modes of Cd2Ta2O7 in the same temperature region as for Cd2Nb2O7 does not exclude an existence of slow relaxation processes and disordered state in this pyrochlore as well.