• Kolpakova, N. N.
• Nawrocik, Wojciech
• Syrnikov, P. P.
• Czarnecki, Piotr
• Szczepanska, L.
• Lebedev, A. O.
• Perrot, C.

Abstract

<p>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 Cd_1.5Fe_0.5²⁺Nb₂O₇ and Cd_1.5Fe_0.5³⁺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 T_C=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 (T_f=183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below T_f the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd₂Nb₂O₇ dominating far below T_C (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 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-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. © 2001 American Institute of Physics.</p>

Topics

• impedance spectroscopy
• dispersion
• compound
• phase
• x-ray diffraction
• glass
• glass
• forming
• random
• ceramic
• dielectric dispersion