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

The weak-field dielectric dispersion (100 Hz-1.8 GHz) studies both of pure and Fe2+/Fe-3 modified Cd2Nb2O7 ceramics over the temperature range of 90-3 80 K are presented and discussed from the viewpoint of relaxor and glassy p roperties of the system. It is revealed that Cd2Nb2O7 pyrochlore is intoler ant of the addition of 25 mol % Fe2+ or Fe3+ for Cd2+. From the x-ray diffr action analysis, pure Cd2Nb2O7 forms a single-phase pyrochlore, while the c ompositions 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), ex cept for minor amount of parasitic hematite. The novel CdNb2O6 type compoun ds are not ferroelectrics, unlike Cd2Nb2O7. In the latter, at T-C=196 K the dielectric relaxation due to the motion of ferroelectric domain walls driv en by an external ac electric field is observed. A polydispersive dielectri c response of Cd2Nb2O7 around 188 K has characteristics of the relaxor ferr oelectrics with glassy behavior (like PMN). Near the characteristic freezin g temperature of the zero-field-cooled state (T-f=183 K) the dielectric abs orption spectra and the relaxation-time distribution strongly broaden and t end to flatten out, while below T-f the imaginary part of the dielectric pe rmittivity becomes nearly frequency independent. The dielectric response of Cd2Nb2O7 dominating far below T-C (around 150 K) and that of Fe2+/Fe3+ dop ed CdNb2O6 between 90 and 380 K are typical of glass-forming systems at tem perature far above T-glass. 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-a bsorption frequency. It is proposed that although the nature of structural disorder in Cd2Nb2O7 pyrochlore and Fe2+/Fe3+ doped CdNb2O6 columbite is di fferent, in both systems the off-center displacements of the A-site ions ac t as a random field and are responsible for the relaxor and dipolar glass-l ike 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 po larization of the dynamically reoriented O(7th)-Cd-O(7th) dipoles due to dy namical off-center location of Cd ions. (C) 2001 American Institute of Phys ics.