Dielectric relaxation and ferroelectricity in Cd2Nb2O7 pyrochlore

The dielectric dispersion of Cd₂Nb₂O₇ pyrochlore in a weak electric field was studied in a broad frequency range (100 Hz to 13 MHz) using the crystal samples slowly cooled (0.5 K/min) in the temperature interval from 300 to 80 K. As the temperature decreased down to T _c=196 K and T _max～190 K, the dielectric permittivity exhibited deviation from the Curie-Weiss law. It is suggested that this behavior is related to the development of a short-range correlation between microscopic polar regions formed at T→T _max ⁺ . The local order parameter q(T) ～ 〈P _i P _j 〉^1/2 was calculated using the permittivity ε′(T) measured at various frequencies. The variation of this parameter is compared to that of the spontaneous polarization P _s (T) determined from the measurements of a pyroelectric current in the external electric field E _dc =0.95 kV/cm. In the frequency range from 100 Hz to 13 MHz, the dispersion of the dielectric response in the temperature region of 180–192 K is characteristic of a relaxator ferroelectric featuring a glasslike behavior. The parameters of this state were determined, including the activation energy of the polarization fluctuations (E _a ≈0.01 eV), the relaxation rate at T → ∞ (f ₀=1.9×10¹² Hz), and the polarization fluctuation freezing temperature (T _f =183 K). In Cd₂Nb₂O₇ pyrochlore, in contrast to the known relaxator ferroelectrics of the PMN type studied previously, this state coexists with the normal ferroelectric state appearing at T _c.