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

Abstract

The weak-field dielectric dispersion ͑100 Hz-1.8 GHz͒ studies both of pure and Fe 2ϩ /Fe 3 modified Cd 2 Nb 2 O 7 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 In the latter, at T C ϭ196 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 Cd 2 Nb 2 O 7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior ͑like PMN͒. Near the characteristic freezing temperature of the zero-field-cooled state (T f ϭ183 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 Cd 2 Nb 2 O 7 dominating far below T C ͑around 150 K͒ and that of Fe 2ϩ /Fe 3ϩ doped CdNb 2 O 6 between 90 and 380 K are typical of glass-forming systems at temperature far above T glass . The relaxational process is characterized by ͑i͒ a significant frequency dependence of the peak permittivity position, ͑ii͒ non-Arrhenius behavior, and ͑iii͒ 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 Cd 2 Nb 2 O 7 pyrochlore and Fe 2ϩ /Fe 3ϩ doped CdNb 2 O 6 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 GHz͒ observed both in Cd 2 Nb 2 O 7 and its isostructural analog Cd 2 Ta 2 O 7 at RT and higher ͑a centrosymmetric phase͒ is attributed to fluctuations in polarization of the dynamically reoriented O͑7th͒-Cd-O͑7th͒ dipoles due to dynamical off-center location of Cd ions.

Electrical properties of sol–gel derived pyrochlore-type bismuth magnesium niobate Bi2(Mg1/3Nb2/3)2O7 thin films

“…The fitting parameters used in the Vo¨gel-Fulcher equation are summarized in Table 2. The freezing temperature (770 K) for the BMN thin film is much larger than those of CNO [10], BZZN [6], BBN [12] and PMN [13]. This is consistent with the high T m for the BMN thin film.…”

“…From the observed diffused phase transition, which is evident from the reduced 0 and the increased T m with increasing frequencies, the BMN thin films can be considered as a relaxor. The diffused phase transitions were also observed for pyrochlore ferroelectric relaxor Cd 2 Nb 2 O 7 (CNO) ceramics [10], pyrochlore (Bi 3/4 Zn 1/4 ) 2 (Zn 1/4 Me 3/4 ) 2 O 7 (Me ¼ Nb and Ta) (BZZN, BZZT) compounds [1,[6][7][8], and ferroelectric relaxors PMN [11] and BaBi 2 Nb 2 O 9 (BBN) [12]. The observed shift of T m with the frequency in BMN is smaller than that of BBN [12] but larger than that of PMN [13].…”

“…This is consistent with the high T m for the BMN thin film. The activation energy E a (0.178 eV) of the BMN thin film is larger than those of CNO [10] and PFW [15] ceramics and similar to those of BZZN [6,8].…”

Dielectric Relaxation Processes in Relaxor Ferroelectric Cd 2 Nb 2 O 7 Modified with Ni 2+ or Cu 2+

“…On partial substitution of Cu 2+ ions for Cd 2+ (x(Cu) = 0.2) the relaxor properties of Cd 2 Nb 2 O 7 have been observed to enhance [9], yet no methodical studies of the dielectric relaxation processes in the diluted compounds have been carried out. Another specificity of Cd 2 Nb 2 O 7 is that a relaxor state of the system coexists with the ferroelastic (below T s = 205 K) and ferroelectric (below T C = 196 K) ones [8][9][10].…”

A novel photocatalyst and improvement of photocatalytic properties by Cu doping

“…It is known that lots of compounds with A 2 3+ B 2 4+ O 7 pyrochlore structure exhibit antiferroelectric phases or dielectric anomalies [12,13], and series of photocatalysts with pyrochlore have been reported, such as Bi 2 MNbO 7 (M = Al, Ga, In and Fe), Bi 2 RNbO 7 (R = Y, rare earth elements) and Bi 2 MTaO 7 (M = In, Ga, Fe, La and Y). Recently, pyrochlore compounds that belong to ternary system Bi 2 O 3 -ZnO-Me 2 O 5 (Me = Nb and Ta) [14][15][16], which contain octahedral units, have been recognized as potential candidates for capacitor and high-frequency filter applications [17][18][19][20].…”