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2-5 pyrochlore relaxor ferroelectric Cd2Nb2O7 and its Fe2+/Fe3+ modifications

Abstract : The weak-field dielectric dispersion (100 Hz-1.8 GHz) studies both of pure and Fe2+/Fe3 modified Cd2Nb2O7 ceramics over the temperature range of 90-380 K are presented and discussed from the viewpoint of relaxor and glassy properties of the system. It is revealed that Cd2Nb2O7 pyrochlore is intolerant of the addition of 25 mol % Fe2+ or Fe3+ for Cd2+. From the x-ray diffraction analysis, pure Cd2Nb2O7 forms a single-phase pyrochlore, while the compositions Cd1.5Fe0.52+Nb2O7 and Cd1.5Fe0.53+Nb2O7 give CdNb2O6 columbite doped with Fe2+ or Fe3+ on the Cd sites (<8 and <2 mol %, respectively), except for minor amount of parasitic hematite. The novel CdNb2O6 type compounds are not ferroelectrics, unlike Cd2Nb2O7. In the latter, at TC=196 K the dielectric relaxation due to the motion of ferroelectric domain walls driven by an external ac electric field is observed. A polydispersive dielectric response of Cd2Nb2O7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior (like PMN). Near the characteristic freezing temperature of the zero-field-cooled state (Tf=183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below Tf the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd2Nb2O7 dominating far below TC (around 150 K) and that of Fe2+/Fe3+ doped CdNb2O6 between 90 and 380 K are typical of glass-forming systems at temperature far above Tglass. The relaxational process is characterized by (i) a significant frequency dependence of the peak permittivity position, (ii) non-Arrhenius behavior, and (iii) increasing asymmetry of the dielectric absorption spectrum at the low-frequency side with decreasing temperature, without broadening the relaxation-time distribution and freezing the peak-absorption frequency. It is proposed that although the nature of structural disorder in Cd2Nb2O7 pyrochlore and Fe2+/Fe3+ doped CdNb2O6 columbite is different, in both systems the off-center displacements of the A-site ions act as a random field and are responsible for the relaxor and dipolar glass-like behavior upon cooling. The Debye-like HF dielectric relaxation (1 MHz-1.8 GHz) observed both in Cd2Nb2O7 and its isostructural analog Cd2Ta2O7 at RT and higher (a centrosymmetric phase) is attributed to fluctuations in polarization of the dynamically reoriented O(7th)-Cd-O(7th) dipoles due to dynamical off-center location of Cd ions.
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N. N. Kolpakova, P. P. Syrnikov, A. O. Lebedev, P. Czarnecki, W. Nawrocik, et al.. 2-5 pyrochlore relaxor ferroelectric Cd2Nb2O7 and its Fe2+/Fe3+ modifications. Journal of Applied Physics, American Institute of Physics, 2001, 90 (12), pp.6332-9. ⟨10.1063/1.1415540⟩. ⟨hal-00731868⟩

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