Relaxor-like behaviour of pyrochlores

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Dielectric dispersion and Raman scattering studies of Cd2Nb2O7 pyrochlore single crystals in the ferroelectric phase (T < Tc= 196 K < Ts = 205 K) are presented. Below Tc, the behaviour of normalized ε (T) and ε (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 secondorder phase transitions at Ts and Tc is due to ‘overheated’ ferroelectric state. The slow relaxation processes developing on a long-time scale below Tc are ascribed to quasi-nonergodicity of the system long before a glassy state appears (below Tg ≈ 18 K). The intensity of the Raman-active hard modes arising below Tc changes on cooling towards Tg 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 Tg < T < Tc 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 Tc. 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.